Re: [PATCH 1/1] KVM: x86: Update cpuid properly when CR4.OSXAVE or CR4.PKE is changed

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Wei Huang <wei@xxxxxxxxxx> writes:

> The CPUID bits of OSXSAVE (function=0x1) and OSPKE (func=0x7, leaf=0x0)
> allows user apps to detect if OS has set CR4.OSXSAVE or CR4.PKE. KVM is
> supposed to update these CPUID bits when CR4 is updated. Current KVM
> code doesn't handle some special cases when updates come from emulator.
> Here is one example:
>
>   Step 1: guest boots
>   Step 2: guest OS enables XSAVE ==> CR4.OSXSAVE=1 and CPUID.OSXSAVE=1
>   Step 3: guest hot reboot ==> QEMU reset CR4 to 0, but CPUID.OSXAVE==1
>   Step 4: guest os checks CPUID.OSXAVE, detects 1, then executes xgetbv
>
> Step 4 above will cause an #UD and guest crash because guest OS hasn't
> turned on OSXAVE yet. This patch solves the problem by comparing the the
> old_cr4 with cr4. If the related bits have been changed,
> kvm_update_cpuid() needs to be called.
>
> Signed-off-by: Wei Huang <wei@xxxxxxxxxx>
> ---
>  arch/x86/kvm/x86.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 51ecd381793b..daeb5e2176c6 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -7979,6 +7979,7 @@ static int __set_sregs(struct kvm_vcpu *vcpu, struct kvm_sregs *sregs)
>  {
>  	struct msr_data apic_base_msr;
>  	int mmu_reset_needed = 0;
> +	int cpuid_update_needed = 0;
>  	int pending_vec, max_bits, idx;
>  	struct desc_ptr dt;
>  	int ret = -EINVAL;
> @@ -8017,8 +8018,10 @@ static int __set_sregs(struct kvm_vcpu *vcpu, struct kvm_sregs *sregs)
>  	vcpu->arch.cr0 = sregs->cr0;
>  
>  	mmu_reset_needed |= kvm_read_cr4(vcpu) != sregs->cr4;
> +	cpuid_update_needed |= ((kvm_read_cr4(vcpu) ^ sregs->cr4) &
> +				(X86_CR4_OSXSAVE | X86_CR4_PKE));
>  	kvm_x86_ops->set_cr4(vcpu, sregs->cr4);
> -	if (sregs->cr4 & (X86_CR4_OSXSAVE | X86_CR4_PKE))
> +	if (cpuid_update_needed)
>  		kvm_update_cpuid(vcpu);
>  
>  	idx = srcu_read_lock(&vcpu->kvm->srcu);

Reviewed-by: Bandan Das <bsd@xxxxxxxxxx>



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