[v6] context_tracking,x86: Defer some IPIs until a user->kernel transition

[PATCH v6 19/29] KVM: VMX: Mark vmx_l1d_should flush and vmx_l1d_flush_cond keys as allowed in .noinstr

Posted by Valentin Schneider 2 months, 1 week ago

Later commits will cause objtool to warn about static keys being used in
.noinstr sections in order to safely defer instruction patching IPIs
targeted at NOHZ_FULL CPUs.

These keys are used in .noinstr code, and can be modified at runtime
(/proc/kernel/vmx* write). However it is not expected that they will be
flipped during latency-sensitive operations, and thus shouldn't be a source
of interference wrt the text patching IPI.

Mark it to let objtool know not to warn about it.

Reported-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Valentin Schneider <vschneid@redhat.com>
---
 arch/x86/kvm/vmx/vmx.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index aa157fe5b7b31..dce2bd7375ec8 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -204,8 +204,15 @@ module_param(pt_mode, int, S_IRUGO);
 
 struct x86_pmu_lbr __ro_after_init vmx_lbr_caps;
 
-static DEFINE_STATIC_KEY_FALSE(vmx_l1d_should_flush);
-static DEFINE_STATIC_KEY_FALSE(vmx_l1d_flush_cond);
+/*
+ * Both of these static keys end up being used in .noinstr sections, however
+ * they are only modified:
+ * - at init
+ * - from a /proc/kernel/vmx* write
+ * thus during latency-sensitive operations they should remain stable.
+ */
+static DEFINE_STATIC_KEY_FALSE_NOINSTR(vmx_l1d_should_flush);
+static DEFINE_STATIC_KEY_FALSE_NOINSTR(vmx_l1d_flush_cond);
 static DEFINE_MUTEX(vmx_l1d_flush_mutex);
 
 /* Storage for pre module init parameter parsing */
-- 
2.51.0

Re: [PATCH v6 19/29] KVM: VMX: Mark vmx_l1d_should flush and vmx_l1d_flush_cond keys as allowed in .noinstr

Posted by Sean Christopherson 2 months ago

On Fri, Oct 10, 2025, Valentin Schneider wrote:
> Later commits will cause objtool to warn about static keys being used in
> .noinstr sections in order to safely defer instruction patching IPIs
> targeted at NOHZ_FULL CPUs.
> 
> These keys are used in .noinstr code, and can be modified at runtime
> (/proc/kernel/vmx* write). However it is not expected that they will be
> flipped during latency-sensitive operations, and thus shouldn't be a source
> of interference wrt the text patching IPI.
>
> Mark it to let objtool know not to warn about it.

Can you elaborate in the changelog on what will happen if the key is toggle?
IIUC, smp_text_poke_batch_finish() will force IPIs if noinstr code is being
patched.  Even just a small footnote like this:

  Note, smp_text_poke_batch_finish() never defers IPIs if noinstr code is
  being patched, i.e. this is purely about silencing objtool warnings.

to make it clear that there's no bug/race being introduced.

> Reported-by: Josh Poimboeuf <jpoimboe@kernel.org>
> Signed-off-by: Valentin Schneider <vschneid@redhat.com>
> ---
>  arch/x86/kvm/vmx/vmx.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index aa157fe5b7b31..dce2bd7375ec8 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -204,8 +204,15 @@ module_param(pt_mode, int, S_IRUGO);
>  
>  struct x86_pmu_lbr __ro_after_init vmx_lbr_caps;
>  
> -static DEFINE_STATIC_KEY_FALSE(vmx_l1d_should_flush);
> -static DEFINE_STATIC_KEY_FALSE(vmx_l1d_flush_cond);
> +/*
> + * Both of these static keys end up being used in .noinstr sections, however
> + * they are only modified:
> + * - at init
> + * - from a /proc/kernel/vmx* write
> + * thus during latency-sensitive operations they should remain stable.
> + */
> +static DEFINE_STATIC_KEY_FALSE_NOINSTR(vmx_l1d_should_flush);
> +static DEFINE_STATIC_KEY_FALSE_NOINSTR(vmx_l1d_flush_cond);
>  static DEFINE_MUTEX(vmx_l1d_flush_mutex);
>  
>  /* Storage for pre module init parameter parsing */
> -- 
> 2.51.0
>

Re: [PATCH v6 19/29] KVM: VMX: Mark vmx_l1d_should flush and vmx_l1d_flush_cond keys as allowed in .noinstr

Posted by Valentin Schneider 2 months ago

On 13/10/25 17:01, Sean Christopherson wrote:
> On Fri, Oct 10, 2025, Valentin Schneider wrote:
>> Later commits will cause objtool to warn about static keys being used in
>> .noinstr sections in order to safely defer instruction patching IPIs
>> targeted at NOHZ_FULL CPUs.
>>
>> These keys are used in .noinstr code, and can be modified at runtime
>> (/proc/kernel/vmx* write). However it is not expected that they will be
>> flipped during latency-sensitive operations, and thus shouldn't be a source
>> of interference wrt the text patching IPI.
>>
>> Mark it to let objtool know not to warn about it.
>
> Can you elaborate in the changelog on what will happen if the key is toggle?
> IIUC, smp_text_poke_batch_finish() will force IPIs if noinstr code is being
> patched.

Right!

> Even just a small footnote like this:
>
>   Note, smp_text_poke_batch_finish() never defers IPIs if noinstr code is
>   being patched, i.e. this is purely about silencing objtool warnings.
>
> to make it clear that there's no bug/race being introduced.

Good point. How about:

"""
Later commits will cause objtool to warn about static keys being used in
.noinstr sections in order to safely defer instruction patching IPIs
targeted at NOHZ_FULL CPUs.

The VMX keys are used in .noinstr code, and can be modified at runtime
(/proc/kernel/vmx* write). However it is not expected that they will be
flipped during latency-sensitive operations, and thus shouldn't be a source
of interference for NOHZ_FULL CPUs wrt the text patching IPI.

Note, smp_text_poke_batch_finish() never defers IPIs if noinstr code is
being patched, i.e. this is purely to tell objtool we're okay with updates
to that key causing IPIs and to silence the associated objtool warning.
"""