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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