On 3/17/25 12:28, Sean Christopherson wrote:
> On Mon, Mar 17, 2025, Tom Lendacky wrote:
>> On 3/17/25 12:20, Tom Lendacky wrote:
>>> An AP destroy request for a target vCPU is typically followed by an
>>> RMPADJUST to remove the VMSA attribute from the page currently being
>>> used as the VMSA for the target vCPU. This can result in a vCPU that
>>> is about to VMRUN to exit with #VMEXIT_INVALID.
>>>
>>> This usually does not happen as APs are typically sitting in HLT when
>>> being destroyed and therefore the vCPU thread is not running at the time.
>>> However, if HLT is allowed inside the VM, then the vCPU could be about to
>>> VMRUN when the VMSA attribute is removed from the VMSA page, resulting in
>>> a #VMEXIT_INVALID when the vCPU actually issues the VMRUN and causing the
>>> guest to crash. An RMPADJUST against an in-use (already running) VMSA
>>> results in a #NPF for the vCPU issuing the RMPADJUST, so the VMSA
>>> attribute cannot be changed until the VMRUN for target vCPU exits. The
>>> Qemu command line option '-overcommit cpu-pm=on' is an example of allowing
>>> HLT inside the guest.
>>>
>>> Use kvm_test_request() to ensure that the target vCPU sees the AP destroy
>>> request before returning to the initiating vCPU.
>>>
>>> Fixes: e366f92ea99e ("KVM: SEV: Support SEV-SNP AP Creation NAE event")
>>> Signed-off-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
>>
>> Sean,
>>
>> If you're ok with this approach for the fix, this patch may need to be
>> adjusted given your series around AP creation fixes, unless you want to
>> put this as an early patch in your series. Let me know what you'd like
>> to do.
>
> This is unsafe as it requires userspace to do KVM_RUN _and_ for the vCPU to get
> far enough along to consume the request.
>
> Very off-the-cuff, but I assume KVM_REQ_UPDATE_PROTECTED_GUEST_STATE just needs
> to be annotated with KVM_REQUEST_WAIT.
Ok, nice. I wasn't sure if KVM_REQUEST_WAIT would be appropriate here.
This is much simpler. Let me test it out and resend if everything goes ok.
Thanks,
Tom
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 04e6c5604bc3..67abfe97c600 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -124,7 +124,8 @@
> KVM_ARCH_REQ_FLAGS(31, KVM_REQUEST_WAIT | KVM_REQUEST_NO_WAKEUP)
> #define KVM_REQ_HV_TLB_FLUSH \
> KVM_ARCH_REQ_FLAGS(32, KVM_REQUEST_WAIT | KVM_REQUEST_NO_WAKEUP)
> -#define KVM_REQ_UPDATE_PROTECTED_GUEST_STATE KVM_ARCH_REQ(34)
> +#define KVM_REQ_UPDATE_PROTECTED_GUEST_STATE \
> + KVM_ARCH_REQ_FLAGS(34, KVM_REQUEST_WAIT)
>
> #define CR0_RESERVED_BITS \
> (~(unsigned long)(X86_CR0_PE | X86_CR0_MP | X86_CR0_EM | X86_CR0_TS \
>
>
