KVM: SVM: Clean up VMRUN=>#VMEXIT assembly

[PATCH 0/8] KVM: SVM: Clean up VMRUN=>#VMEXIT assembly

Posted by Sean Christopherson 1 year, 11 months ago

Clean up SVM's enter/exit assembly code so that it can be compiled
without OBJECT_FILES_NON_STANDARD.  The "standard" __svm_vcpu_run() can't
be made 100% bulletproof, as RBP isn't restored on #VMEXIT, but that's
also the case for __vmx_vcpu_run(), and getting "close enough" is better
than not even trying.

As for SEV-ES, after yet another refresher on swap types, I realized KVM
can simply let the hardware restore registers after #VMEXIT, all that's
missing is storing the current values to the host save area (I learned the
hard way that they are swap Type B, *sigh*).  Unless I'm missing something,
this provides 100% accuracy when using stack frames for unwinding, and
requires less assembly (though probably not fewer code bytes; I didn't check).

In between, build the SEV-ES code iff CONFIG_KVM_AMD_SEV=y, and yank out
"support" for 32-bit kernels, which was unncessarily polluting the code.

I'm pretty sure I actually managed to test all of this, thanks to the SEV-ES
smoke selftests, and a bit of hacking to disable V_SPEC_CTRL, passthrough
SPEC_CTRL unconditionally, and have the selftests W/R SPEC_CTRL from its
guest.

Sean Christopherson (8):
  KVM: SVM: Create a stack frame in __svm_vcpu_run() for unwinding
  KVM: SVM: Wrap __svm_sev_es_vcpu_run() with #ifdef CONFIG_KVM_AMD_SEV
  KVM: SVM: Drop 32-bit "support" from __svm_sev_es_vcpu_run()
  KVM: SVM: Clobber RAX instead of RBX when discarding
    spec_ctrl_intercepted
  KVM: SVM: Save/restore non-volatile GPRs in SEV-ES VMRUN via host save
    area
  KVM: SVM: Save/restore args across SEV-ES VMRUN via host save area
  KVM: SVM: Create a stack frame in __svm_sev_es_vcpu_run()
  KVM: x86: Stop compiling vmenter.S with OBJECT_FILES_NON_STANDARD

 arch/x86/kvm/Makefile      |  4 --
 arch/x86/kvm/svm/svm.c     | 17 ++++---
 arch/x86/kvm/svm/svm.h     |  3 +-
 arch/x86/kvm/svm/vmenter.S | 97 +++++++++++++++++---------------------
 4 files changed, 56 insertions(+), 65 deletions(-)


base-commit: ec1e3d33557babed2c2c2c7da6e84293c2f56f58
-- 
2.44.0.rc0.258.g7320e95886-goog

Re: [PATCH 0/8] KVM: SVM: Clean up VMRUN=>#VMEXIT assembly

Posted by Sean Christopherson 1 year, 10 months ago

On Fri, 23 Feb 2024 12:42:25 -0800, Sean Christopherson wrote:
> Clean up SVM's enter/exit assembly code so that it can be compiled
> without OBJECT_FILES_NON_STANDARD.  The "standard" __svm_vcpu_run() can't
> be made 100% bulletproof, as RBP isn't restored on #VMEXIT, but that's
> also the case for __vmx_vcpu_run(), and getting "close enough" is better
> than not even trying.
> 
> As for SEV-ES, after yet another refresher on swap types, I realized KVM
> can simply let the hardware restore registers after #VMEXIT, all that's
> missing is storing the current values to the host save area (I learned the
> hard way that they are swap Type B, *sigh*).  Unless I'm missing something,
> this provides 100% accuracy when using stack frames for unwinding, and
> requires less assembly (though probably not fewer code bytes; I didn't check).
> 
> [...]

Applied to kvm-x86 svm, thanks!

[1/8] KVM: SVM: Create a stack frame in __svm_vcpu_run() for unwinding
      https://github.com/kvm-x86/linux/commit/19597a71a0c8
[2/8] KVM: SVM: Wrap __svm_sev_es_vcpu_run() with #ifdef CONFIG_KVM_AMD_SEV
      https://github.com/kvm-x86/linux/commit/7774c8f32e99
[3/8] KVM: SVM: Drop 32-bit "support" from __svm_sev_es_vcpu_run()
      https://github.com/kvm-x86/linux/commit/331282fdb15e
[4/8] KVM: SVM: Clobber RAX instead of RBX when discarding spec_ctrl_intercepted
      https://github.com/kvm-x86/linux/commit/87e8e360a05f
[5/8] KVM: SVM: Save/restore non-volatile GPRs in SEV-ES VMRUN via host save area
      https://github.com/kvm-x86/linux/commit/c92be2fd8edf
[6/8] KVM: SVM: Save/restore args across SEV-ES VMRUN via host save area
      https://github.com/kvm-x86/linux/commit/adac42bf42c1
[7/8] KVM: SVM: Create a stack frame in __svm_sev_es_vcpu_run()
      https://github.com/kvm-x86/linux/commit/4367a75887ec
[8/8] KVM: x86: Stop compiling vmenter.S with OBJECT_FILES_NON_STANDARD
      https://github.com/kvm-x86/linux/commit/27ca867042af

--
https://github.com/kvm-x86/linux/tree/next

Re: [PATCH 0/8] KVM: SVM: Clean up VMRUN=>#VMEXIT assembly

Posted by Paolo Bonzini 1 year, 9 months ago

On Wed, Apr 10, 2024 at 2:23 AM Sean Christopherson <seanjc@google.com> wrote:
> Applied to kvm-x86 svm, thanks!
>
> [1/8] KVM: SVM: Create a stack frame in __svm_vcpu_run() for unwinding
>       https://github.com/kvm-x86/linux/commit/19597a71a0c8
> [2/8] KVM: SVM: Wrap __svm_sev_es_vcpu_run() with #ifdef CONFIG_KVM_AMD_SEV
>       https://github.com/kvm-x86/linux/commit/7774c8f32e99
> [3/8] KVM: SVM: Drop 32-bit "support" from __svm_sev_es_vcpu_run()
>       https://github.com/kvm-x86/linux/commit/331282fdb15e
> [4/8] KVM: SVM: Clobber RAX instead of RBX when discarding spec_ctrl_intercepted
>       https://github.com/kvm-x86/linux/commit/87e8e360a05f
> [5/8] KVM: SVM: Save/restore non-volatile GPRs in SEV-ES VMRUN via host save area
>       https://github.com/kvm-x86/linux/commit/c92be2fd8edf
> [6/8] KVM: SVM: Save/restore args across SEV-ES VMRUN via host save area
>       https://github.com/kvm-x86/linux/commit/adac42bf42c1
> [7/8] KVM: SVM: Create a stack frame in __svm_sev_es_vcpu_run()
>       https://github.com/kvm-x86/linux/commit/4367a75887ec
> [8/8] KVM: x86: Stop compiling vmenter.S with OBJECT_FILES_NON_STANDARD
>       https://github.com/kvm-x86/linux/commit/27ca867042af

Do we perhaps want this in 6.9 because of the issues that was reported
with objtool?

Paolo

Re: [PATCH 0/8] KVM: SVM: Clean up VMRUN=>#VMEXIT assembly

Posted by Tom Lendacky 1 year, 11 months ago

On 2/23/24 14:42, Sean Christopherson wrote:
> Clean up SVM's enter/exit assembly code so that it can be compiled
> without OBJECT_FILES_NON_STANDARD.  The "standard" __svm_vcpu_run() can't
> be made 100% bulletproof, as RBP isn't restored on #VMEXIT, but that's
> also the case for __vmx_vcpu_run(), and getting "close enough" is better
> than not even trying.
> 
> As for SEV-ES, after yet another refresher on swap types, I realized KVM
> can simply let the hardware restore registers after #VMEXIT, all that's
> missing is storing the current values to the host save area (I learned the
> hard way that they are swap Type B, *sigh*).  Unless I'm missing something,
> this provides 100% accuracy when using stack frames for unwinding, and
> requires less assembly (though probably not fewer code bytes; I didn't check).
> 
> In between, build the SEV-ES code iff CONFIG_KVM_AMD_SEV=y, and yank out
> "support" for 32-bit kernels, which was unncessarily polluting the code.
> 
> I'm pretty sure I actually managed to test all of this, thanks to the SEV-ES
> smoke selftests, and a bit of hacking to disable V_SPEC_CTRL, passthrough
> SPEC_CTRL unconditionally, and have the selftests W/R SPEC_CTRL from its
> guest.
> 
> Sean Christopherson (8):
>    KVM: SVM: Create a stack frame in __svm_vcpu_run() for unwinding
>    KVM: SVM: Wrap __svm_sev_es_vcpu_run() with #ifdef CONFIG_KVM_AMD_SEV
>    KVM: SVM: Drop 32-bit "support" from __svm_sev_es_vcpu_run()
>    KVM: SVM: Clobber RAX instead of RBX when discarding
>      spec_ctrl_intercepted
>    KVM: SVM: Save/restore non-volatile GPRs in SEV-ES VMRUN via host save
>      area
>    KVM: SVM: Save/restore args across SEV-ES VMRUN via host save area
>    KVM: SVM: Create a stack frame in __svm_sev_es_vcpu_run()
>    KVM: x86: Stop compiling vmenter.S with OBJECT_FILES_NON_STANDARD
> 
>   arch/x86/kvm/Makefile      |  4 --
>   arch/x86/kvm/svm/svm.c     | 17 ++++---
>   arch/x86/kvm/svm/svm.h     |  3 +-
>   arch/x86/kvm/svm/vmenter.S | 97 +++++++++++++++++---------------------
>   4 files changed, 56 insertions(+), 65 deletions(-)

Nice cleanup, thanks!

For the series:
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>

> 
> 
> base-commit: ec1e3d33557babed2c2c2c7da6e84293c2f56f58