linux-kernel - Re: [CFT PATCH v2 2/2] KVM: x86: support XSAVES usage in the host

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Date:	Thu, 04 Dec 2014 14:43:27 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	linux-kernel@...r.kernel.org
Cc:	kvm@...r.kernel.org
Subject: Re: [CFT PATCH v2 2/2] KVM: x86: support XSAVES usage in the host



On 03/12/2014 19:45, Radim Krčmář wrote:
> 2014-12-03 15:26+0100, Paolo Bonzini:
>>
>>
>> On 03/12/2014 15:23, Nadav Amit wrote:
>>> I think it is better just to replace the last line with:
>>>
>>> *(u64 *)(dest + XSAVE_HDR_OFFSET) = xsave->xsave_hdr.xstate_bv
> 
> Yeah, or we can use this value for xstate_bv to save some copying too,
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 19e5e8f..ba2b7bd 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3137,7 +3137,7 @@ static int kvm_vcpu_ioctl_x86_set_debugregs(struct kvm_vcpu *vcpu,
>  static void fill_xsave(u8 *dest, struct kvm_vcpu *vcpu)
>  {
>  	struct xsave_struct *xsave = &vcpu->arch.guest_fpu.state->xsave;
> -	u64 xstate_bv = vcpu->arch.guest_supported_xcr0 | XSTATE_FPSSE;
> +	u64 xstate_bv = xsave->xsave_hdr.xstate_bv;

Also good.

>  	u64 valid;
>  
>  	/*
> 
>> Right, this matches
>>
>>         u64 xstate_bv = *(u64 *)(src + XSAVE_HDR_OFFSET);
>> 	...
>>         xsave->xsave_hdr.xstate_bv = xstate_bv;
>>
>> in load_xsave.
> 
> Btw, we don't care about crashers from userspace?

We do, but they're taken care of by

                if (xstate_bv & ~kvm_supported_xcr0())
                        return -EINVAL;

in kvm_vcpu_ioctl_x86_set_xsave.  kvm_supported_xcr0 is always a subset
of host_xcr0, hence:

>  	/* Set XSTATE_BV and possibly XCOMP_BV.  */
>  	xsave->xsave_hdr.xstate_bv = xstate_bv;
> -	if (cpu_has_xsaves)
> +	if (cpu_has_xsaves) {
>  		xsave->xsave_hdr.xcomp_bv = host_xcr0 | XSTATE_COMPACTION_ENABLED;
> +		xsave->xsave_hdr.xstate_bv &= xsave->xsave_hdr.xcomp_bv;

this is not zeroing any bit.  Thanks for thinking about it, though. :)

Paolo

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