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[PATCH 4/4] KVM: TDX: Always honor guest PAT on TDX enabled platforms

Paolo Bonzini posted 4 patches 11 months, 2 weeks ago
There is a newer version of this series
[PATCH 4/4] KVM: TDX: Always honor guest PAT on TDX enabled platforms
Posted by Paolo Bonzini 11 months, 2 weeks ago 
From: Yan Zhao <yan.y.zhao@intel.com>

Always honor guest PAT in KVM-managed EPTs on TDX enabled platforms by
making self-snoop feature a hard dependency for TDX and making quirk
KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT not a valid quirk once TDX is enabled.

The quirk KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT only affects memory type of
KVM-managed EPTs. For the TDX-module-managed private EPT, memory type is
always forced to WB now.

Honoring guest PAT in KVM-managed EPTs ensures KVM does not invoke
kvm_zap_gfn_range() when attaching/detaching non-coherent DMA devices;
this would cause mirrored EPTs for TDs to be zapped, as well as incorrect
zapping of the private EPT that is managed by the TDX module.

As a new platform, TDX always comes with self-snoop feature supported and has
no worry to break old not-well-written yet unmodifiable guests. So, simply
force-disable the KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT quirk for TDX VMs.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
Message-ID: <20250224071039.31511-1-yan.y.zhao@intel.com>
[Use disabled_quirks instead of supported_quirks. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 arch/x86/kvm/vmx/tdx.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index b6f6f6e2f02e..4450fd99cb4c 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -624,6 +624,7 @@ int tdx_vm_init(struct kvm *kvm)
 
 	kvm->arch.has_protected_state = true;
 	kvm->arch.has_private_mem = true;
+	kvm->arch.disabled_quirks |= KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT;
 
 	/*
 	 * Because guest TD is protected, VMM can't parse the instruction in TD.
@@ -3470,6 +3471,11 @@ int __init tdx_bringup(void)
 		goto success_disable_tdx;
 	}
 
+	if (!cpu_feature_enabled(X86_FEATURE_SELFSNOOP)) {
+		pr_err("Self-snoop is required for TDX\n");
+		goto success_disable_tdx;
+	}
+
 	if (!cpu_feature_enabled(X86_FEATURE_TDX_HOST_PLATFORM)) {
 		pr_err("tdx: no TDX private KeyIDs available\n");
 		goto success_disable_tdx;
-- 
2.43.5
Re: [PATCH 4/4] KVM: TDX: Always honor guest PAT on TDX enabled platforms
Posted by Yan Zhao 11 months, 1 week ago 
On Sat, Mar 01, 2025 at 02:34:28AM -0500, Paolo Bonzini wrote:
> From: Yan Zhao <yan.y.zhao@intel.com>
> 
> Always honor guest PAT in KVM-managed EPTs on TDX enabled platforms by
> making self-snoop feature a hard dependency for TDX and making quirk
> KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT not a valid quirk once TDX is enabled.
> 
> The quirk KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT only affects memory type of
> KVM-managed EPTs. For the TDX-module-managed private EPT, memory type is
> always forced to WB now.
> 
> Honoring guest PAT in KVM-managed EPTs ensures KVM does not invoke
> kvm_zap_gfn_range() when attaching/detaching non-coherent DMA devices;
> this would cause mirrored EPTs for TDs to be zapped, as well as incorrect
> zapping of the private EPT that is managed by the TDX module.
> 
> As a new platform, TDX always comes with self-snoop feature supported and has
> no worry to break old not-well-written yet unmodifiable guests. So, simply
> force-disable the KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT quirk for TDX VMs.
> 
> Suggested-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
> Message-ID: <20250224071039.31511-1-yan.y.zhao@intel.com>
> [Use disabled_quirks instead of supported_quirks. - Paolo]
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  arch/x86/kvm/vmx/tdx.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> index b6f6f6e2f02e..4450fd99cb4c 100644
> --- a/arch/x86/kvm/vmx/tdx.c
> +++ b/arch/x86/kvm/vmx/tdx.c
> @@ -624,6 +624,7 @@ int tdx_vm_init(struct kvm *kvm)
>  
>  	kvm->arch.has_protected_state = true;
>  	kvm->arch.has_private_mem = true;
> +	kvm->arch.disabled_quirks |= KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT;
Though the quirk is disabled by default in KVM in tdx_vm_init() for TDs, the
kvm->arch.disabled_quirks can be overwritten by a userspace specified value in
kvm_vm_ioctl_enable_cap().
"kvm->arch.disabled_quirks = cap->args[0] & kvm_caps.supported_quirks;"

So, when an old userspace tries to disable other quirks on this new KVM, it may
accidentally turn KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT into enabled for TDs, which
would cause SEPT being zapped when (de)attaching non-coherent devices.

Could we force KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT to be disabled for TDs?

e.g.

tdx_vm_init
   kvm->arch.always_disabled_quirks |= KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT;

static inline bool kvm_check_has_quirk(struct kvm *kvm, u64 quirk)
{
        WARN_ON_ONCE(kvm->arch.always_disabled_quirk & kvm_caps.force_enabled_quirks);

        u64 disabled_quirks = kvm->arch.always_disabled_quirk | kvm->arch.disabled_quirks;
        return !(disabled_quirks & quirk) |
               (kvm_caps.force_enabled_quirks & quirk);
}

>  
>  	/*
>  	 * Because guest TD is protected, VMM can't parse the instruction in TD.
> @@ -3470,6 +3471,11 @@ int __init tdx_bringup(void)
>  		goto success_disable_tdx;
>  	}
>  
> +	if (!cpu_feature_enabled(X86_FEATURE_SELFSNOOP)) {
> +		pr_err("Self-snoop is required for TDX\n");
> +		goto success_disable_tdx;
> +	}
> +
>  	if (!cpu_feature_enabled(X86_FEATURE_TDX_HOST_PLATFORM)) {
>  		pr_err("tdx: no TDX private KeyIDs available\n");
>  		goto success_disable_tdx;
> -- 
> 2.43.5
>
Re: [PATCH 4/4] KVM: TDX: Always honor guest PAT on TDX enabled platforms
Posted by Paolo Bonzini 11 months, 1 week ago 
On 3/3/25 02:30, Yan Zhao wrote:
>>   	kvm->arch.has_protected_state = true;
>>   	kvm->arch.has_private_mem = true;
>> +	kvm->arch.disabled_quirks |= KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT;
> Though the quirk is disabled by default in KVM in tdx_vm_init() for TDs, the
> kvm->arch.disabled_quirks can be overwritten by a userspace specified value in
> kvm_vm_ioctl_enable_cap().
> "kvm->arch.disabled_quirks = cap->args[0] & kvm_caps.supported_quirks;"
> 
> So, when an old userspace tries to disable other quirks on this new KVM, it may
> accidentally turn KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT into enabled for TDs, which
> would cause SEPT being zapped when (de)attaching non-coherent devices.

Yeah, sorry about that - Xiaoyao also pointed it out and I should have 
noticed it---or marked the patches as RFC which they were.

> Could we force KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT to be disabled for TDs?
> 
> e.g.
> 
> tdx_vm_init
>     kvm->arch.always_disabled_quirks |= KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT;
> 
> static inline bool kvm_check_has_quirk(struct kvm *kvm, u64 quirk)
> {
>          WARN_ON_ONCE(kvm->arch.always_disabled_quirk & kvm_caps.force_enabled_quirks);
> 
>          u64 disabled_quirks = kvm->arch.always_disabled_quirk | kvm->arch.disabled_quirks;
>          return !(disabled_quirks & quirk) |
>                 (kvm_caps.force_enabled_quirks & quirk);
> }

We can change KVM_ENABLE_CAP(KVM_X86_DISABLE_QUIRKS), as well as 
QUIRKS2, to use "|=" instead of "=".

While this is technically a change in the API, the current 
implementation is just awful and I hope that no one is relying on it! 
This way, the "always_disabled_quirks" are not needed.

If the "|=" idea doesn't work out I agree that 
kvm->arch.always_disabled_quirk is needed.

Sending v3 shortly...

Paolo
Re: [PATCH 4/4] KVM: TDX: Always honor guest PAT on TDX enabled platforms
Posted by Yan Zhao 11 months, 1 week ago 
On Mon, Mar 03, 2025 at 05:14:55PM +0100, Paolo Bonzini wrote:
> On 3/3/25 02:30, Yan Zhao wrote:
> > >   	kvm->arch.has_protected_state = true;
> > >   	kvm->arch.has_private_mem = true;
> > > +	kvm->arch.disabled_quirks |= KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT;
> > Though the quirk is disabled by default in KVM in tdx_vm_init() for TDs, the
> > kvm->arch.disabled_quirks can be overwritten by a userspace specified value in
> > kvm_vm_ioctl_enable_cap().
> > "kvm->arch.disabled_quirks = cap->args[0] & kvm_caps.supported_quirks;"
> > 
> > So, when an old userspace tries to disable other quirks on this new KVM, it may
> > accidentally turn KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT into enabled for TDs, which
> > would cause SEPT being zapped when (de)attaching non-coherent devices.
> 
> Yeah, sorry about that - Xiaoyao also pointed it out and I should have
> noticed it---or marked the patches as RFC which they were.
> 
> > Could we force KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT to be disabled for TDs?
> > 
> > e.g.
> > 
> > tdx_vm_init
> >     kvm->arch.always_disabled_quirks |= KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT;
> > 
> > static inline bool kvm_check_has_quirk(struct kvm *kvm, u64 quirk)
> > {
> >          WARN_ON_ONCE(kvm->arch.always_disabled_quirk & kvm_caps.force_enabled_quirks);
> > 
> >          u64 disabled_quirks = kvm->arch.always_disabled_quirk | kvm->arch.disabled_quirks;
> >          return !(disabled_quirks & quirk) |
> >                 (kvm_caps.force_enabled_quirks & quirk);
> > }
> 
> We can change KVM_ENABLE_CAP(KVM_X86_DISABLE_QUIRKS), as well as QUIRKS2, to
> use "|=" instead of "=".
> 
> While this is technically a change in the API, the current implementation is
> just awful and I hope that no one is relying on it! This way, the
I think QEMU is not relying on it.

I considered making this change while writing the quirk SLOT_ZAP_ALL but gave it
up, thinking it might allow users to re-enable a quirk later on.

I'm glad you also see it as a bug:)

> "always_disabled_quirks" are not needed.
> 
> If the "|=" idea doesn't work out I agree that
> kvm->arch.always_disabled_quirk is needed.
> 
> Sending v3 shortly...
Thanks!
Re: [PATCH 4/4] KVM: TDX: Always honor guest PAT on TDX enabled platforms
Posted by Xiaoyao Li 11 months, 1 week ago 
On 3/1/2025 3:34 PM, Paolo Bonzini wrote:
> From: Yan Zhao <yan.y.zhao@intel.com>
> 
> Always honor guest PAT in KVM-managed EPTs on TDX enabled platforms by
> making self-snoop feature a hard dependency for TDX and making quirk
> KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT not a valid quirk once TDX is enabled.
> 
> The quirk KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT only affects memory type of
> KVM-managed EPTs. For the TDX-module-managed private EPT, memory type is
> always forced to WB now.
> 
> Honoring guest PAT in KVM-managed EPTs ensures KVM does not invoke
> kvm_zap_gfn_range() when attaching/detaching non-coherent DMA devices;
> this would cause mirrored EPTs for TDs to be zapped, as well as incorrect
> zapping of the private EPT that is managed by the TDX module.
> 
> As a new platform, TDX always comes with self-snoop feature supported and has
> no worry to break old not-well-written yet unmodifiable guests. So, simply
> force-disable the KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT quirk for TDX VMs.
> 
> Suggested-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
> Message-ID: <20250224071039.31511-1-yan.y.zhao@intel.com>
> [Use disabled_quirks instead of supported_quirks. - Paolo]
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>   arch/x86/kvm/vmx/tdx.c | 6 ++++++
>   1 file changed, 6 insertions(+)
> 
> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> index b6f6f6e2f02e..4450fd99cb4c 100644
> --- a/arch/x86/kvm/vmx/tdx.c
> +++ b/arch/x86/kvm/vmx/tdx.c
> @@ -624,6 +624,7 @@ int tdx_vm_init(struct kvm *kvm)
>   
>   	kvm->arch.has_protected_state = true;
>   	kvm->arch.has_private_mem = true;
> +	kvm->arch.disabled_quirks |= KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT;

This doesn't present userspace from dropping the 
KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT bit by updating
kvm->arch.disabled_quirk via KVM_CAP_DISABLE_QUIRKS.

I think we can make inapplicable_quirks per VM in Patch 1 and set

     kvm->arch.inapplicable_quirks |= KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT;

for TDX VMs.

>   
>   	/*
>   	 * Because guest TD is protected, VMM can't parse the instruction in TD.
> @@ -3470,6 +3471,11 @@ int __init tdx_bringup(void)
>   		goto success_disable_tdx;
>   	}
>   
> +	if (!cpu_feature_enabled(X86_FEATURE_SELFSNOOP)) {
> +		pr_err("Self-snoop is required for TDX\n");
> +		goto success_disable_tdx;
> +	}
> +
>   	if (!cpu_feature_enabled(X86_FEATURE_TDX_HOST_PLATFORM)) {
>   		pr_err("tdx: no TDX private KeyIDs available\n");
>   		goto success_disable_tdx;

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