linux-kernel - Re: [PATCH v2 2/3] KVM: x86: prevent setting unsupported XSAVE states

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Date:	Mon, 23 Sep 2013 17:24:20 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Gleb Natapov <gleb@...hat.com>
CC:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/3] KVM: x86: prevent setting unsupported XSAVE states

Il 23/09/2013 17:09, Gleb Natapov ha scritto:
> On Mon, Sep 23, 2013 at 12:22:37PM +0200, Paolo Bonzini wrote:
>>>> @@ -6940,6 +6948,9 @@ int kvm_arch_vcpu_init(struct kvm_vcpu *vcpu)
>>>>  
>>>>  	vcpu->arch.ia32_tsc_adjust_msr = 0x0;
>>>>  	vcpu->arch.pv_time_enabled = false;
>>>> +
>>>> +	vcpu->arch.supported_xcr0 = XSTATE_FPSSE;
>>>> +
>>> Why is this needed? It will make make __kvm_set_xcr() succeed if attempt
>>> is made to set SSE bit when it is not supported in cpuid. This may not
>>> be an issue in practice, but for clarity it is better for supported_xcr0
>>> to contain only what is supported in guest's cpuid bits and handle the
>>> fact that FP/SSE state should always be copied to/from userspace in
>>> kvm_vcpu_ioctl_x86_(set|get)_xsave functions.
>>
>> I don't think it makes sense to disable SSE but not XSAVE.  Linux even
>> has this:
>>
>>         if ((pcntxt_mask & XSTATE_FPSSE) != XSTATE_FPSSE) {
>>                 pr_err("FP/SSE not shown under xsave features 0x%llx\n",
>>                        pcntxt_mask);
>>                 BUG();
>>         }
>>
> That's why I said it may not be an issue in practice. What makes the
> code confusing to me is that it is not clear what .supported_xcr0
> contains. Why not make it contain only guest cpuid bits and use
> XSTATE_FPSSE explicitly in get_xsave. I mean drop this from next patch:
> 
>         vcpu->arch.supported_xcr0 =
> -               (best->eax | ((u64)best->edx << 32)) &
> +               (best->eax | ((u64)best->edx << 32) | XSTATE_FPSSE) &
>                 host_xcr0 & KVM_SUPPORTED_XCR0;
> 
> And change
> *(u64 *)&guest_xsave->region[XSAVE_HDR_OFFSET / sizeof(u32)] !=  vcpu->arch.supported_xcr0;
> to
> *(u64 *)&guest_xsave->region[XSAVE_HDR_OFFSET / sizeof(u32)] != (vcpu->arch.supported_xcr0 | XSTATE_FPSSE);

Ok.

> 
>> I preferred this to adding dubiously-testable code that probed CPUID for
>> SSE support (or should I check FXSR instead?)

I didn't understand you wanted the "| XSTATE_FPSSE" moved, I thought you
wanted to check for SSE bits in userspace (similar to AVX).  I wasn't
sure whether to tie this to the SSE bit, or to FXSR.  But what you
propose makes sense, I'll test it.

Paolo
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