linux-kernel - Re: [PATCH v1 1/2] x86/kvm/fpu: Mask guest fpstate->xfeatures with guest_supported

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Date:   Mon, 7 Feb 2022 23:59:45 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Leonardo Bras Soares Passos <leobras@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 1/2] x86/kvm/fpu: Mask guest fpstate->xfeatures with
 guest_supported_xcr0

On 2/7/22 23:45, Leonardo Bras Soares Passos wrote:
> On Mon, Feb 7, 2022 at 6:00 PM Paolo Bonzini <pbonzini@...hat.com> wrote:
>>
>> On 2/7/22 21:24, Leonardo Bras Soares Passos wrote:
>>>> With this patch,
>>>> we have to reason about the effect of calling KVM_SET_CPUID2 twice calls
>>>> back to back.  I think an "&=" would be wrong in that case.
>>>
>>> So, you suggest something like this ?
>>>
>>> vcpu->arch.guest_fpu.fpstate->xfeatures =
>>>          fpu_user_cfg.default_features & vcpu->arch.guest_supported_xcr0;
>>>
>>
>> Yes, but you need to change user_xfeatures instead of xfeatures.
>> KVM_GET_XSAVE and KVM_SET_XSAVE will take it into account automatically:
>>
>> - KVM_GET_XSAVE: fpu_copy_guest_fpstate_to_uabi -> __copy_xstate_to_uabi_buf
>>
>> - KVM_SET_XSAVE: fpu_copy_uabi_to_guest_fpstate ->
>> copy_uabi_from_kernel_to_xstate -> copy_uabi_to_xstate ->
>> validate_user_xstate_buffer
> 
> 
> Ok, I understand how this replaces patch 2/2, so no issue on that.
> 
> About patch 1/2,  you suggest that instead of fixing what we save in
> the regs buffer, we fix only what we want to return to the user when
> they call KVM_GET_XSAVE, is that correct?

Yes, exactly.

Paolo