* [PATCH] KVM: x86: Handle MXCSR in kvm_arch_vcpu_ioctl_{get,set}_fpu.
@ 2022-09-17 0:09 Justinien Bouron
2022-09-20 21:33 ` Sean Christopherson
0 siblings, 1 reply; 2+ messages in thread
From: Justinien Bouron @ 2022-09-17 0:09 UTC (permalink / raw)
To: Sean Christopherson, Paolo Bonzini, Thomas Gleixner, Ingo Molnar,
Borislav Petkov, Dave Hansen, x86, H. Peter Anvin
Cc: Justinien Bouron, kvm, linux-kernel
kvm_arch_vcpu_ioctl_get_fpu does not set the mxcsr in the kvm_fpu
struct; conversely kvm_arch_vcpu_ioctl_set_fpu does not set the mxcsr
value in the fxregs_state struct of the vcpu.
This leads to the KVM_GET_FPU ioctl returning 0 as the mxcsr value,
regardless of the actual value on the vcpu; while KVM_SET_FPU leaves the
MXCSR on the vcpu untouched.
Fix kvm_arch_vcpu_ioctl_{get,set}_fpu to properly handle MXCSR.
Signed-off-by: Justinien Bouron <justinien.bouron@gmail.com>
---
arch/x86/kvm/x86.c | 2 ++
1 file changed, 2 insertions(+)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 43a6a7efc6ec..c33a2599a497 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -11462,6 +11462,7 @@ int kvm_arch_vcpu_ioctl_get_fpu(struct kvm_vcpu *vcpu, struct kvm_fpu *fpu)
fpu->last_ip = fxsave->rip;
fpu->last_dp = fxsave->rdp;
memcpy(fpu->xmm, fxsave->xmm_space, sizeof(fxsave->xmm_space));
+ fpu->mxcsr = fxsave->mxcsr;
vcpu_put(vcpu);
return 0;
@@ -11486,6 +11487,7 @@ int kvm_arch_vcpu_ioctl_set_fpu(struct kvm_vcpu *vcpu, struct kvm_fpu *fpu)
fxsave->rip = fpu->last_ip;
fxsave->rdp = fpu->last_dp;
memcpy(fxsave->xmm_space, fpu->xmm, sizeof(fxsave->xmm_space));
+ fxsave->mxcsr = fpu->mxcsr;
vcpu_put(vcpu);
return 0;
--
2.25.1
^ permalink raw reply related [flat|nested] 2+ messages in thread
* Re: [PATCH] KVM: x86: Handle MXCSR in kvm_arch_vcpu_ioctl_{get,set}_fpu.
2022-09-17 0:09 [PATCH] KVM: x86: Handle MXCSR in kvm_arch_vcpu_ioctl_{get,set}_fpu Justinien Bouron
@ 2022-09-20 21:33 ` Sean Christopherson
0 siblings, 0 replies; 2+ messages in thread
From: Sean Christopherson @ 2022-09-20 21:33 UTC (permalink / raw)
To: Justinien Bouron
Cc: Paolo Bonzini, Thomas Gleixner, Ingo Molnar, Borislav Petkov,
Dave Hansen, x86, H. Peter Anvin, kvm, linux-kernel
On Sat, Sep 17, 2022, Justinien Bouron wrote:
> kvm_arch_vcpu_ioctl_get_fpu does not set the mxcsr in the kvm_fpu
> struct; conversely kvm_arch_vcpu_ioctl_set_fpu does not set the mxcsr
> value in the fxregs_state struct of the vcpu.
> This leads to the KVM_GET_FPU ioctl returning 0 as the mxcsr value,
> regardless of the actual value on the vcpu; while KVM_SET_FPU leaves the
> MXCSR on the vcpu untouched.
>
> Fix kvm_arch_vcpu_ioctl_{get,set}_fpu to properly handle MXCSR.
>
> Signed-off-by: Justinien Bouron <justinien.bouron@gmail.com>
> ---
> arch/x86/kvm/x86.c | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 43a6a7efc6ec..c33a2599a497 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -11462,6 +11462,7 @@ int kvm_arch_vcpu_ioctl_get_fpu(struct kvm_vcpu *vcpu, struct kvm_fpu *fpu)
> fpu->last_ip = fxsave->rip;
> fpu->last_dp = fxsave->rdp;
> memcpy(fpu->xmm, fxsave->xmm_space, sizeof(fxsave->xmm_space));
> + fpu->mxcsr = fxsave->mxcsr;
>
> vcpu_put(vcpu);
> return 0;
> @@ -11486,6 +11487,7 @@ int kvm_arch_vcpu_ioctl_set_fpu(struct kvm_vcpu *vcpu, struct kvm_fpu *fpu)
> fxsave->rip = fpu->last_ip;
> fxsave->rdp = fpu->last_dp;
> memcpy(fxsave->xmm_space, fpu->xmm, sizeof(fxsave->xmm_space));
> + fxsave->mxcsr = fpu->mxcsr;
The incoming MXCSR needs to be vetted, e.g. see fpu_copy_uabi_to_guest_fpstate().
That said, this code is incredibly ancient and has been obsolete for years. I
wonder if we can get away with deprecating KVM_{G,S}ET_FPU on x86 and leaving the
broken behavior as-is.
^ permalink raw reply [flat|nested] 2+ messages in thread
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2022-09-20 21:33 ` Sean Christopherson
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