linux-kernel - Re: [PATCH v1] kvm: x86: Revise guest_fpu xcomp

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Date:   Mon, 22 Feb 2021 11:21:06 +0800
From:   "Liu, Jing2" <jing2.liu@...ux.intel.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Dave Hansen <dave.hansen@...el.com>
Cc:     pbonzini@...hat.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>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] kvm: x86: Revise guest_fpu xcomp_bv field



On 2/9/2021 1:24 AM, Sean Christopherson wrote:
> On Mon, Feb 08, 2021, Dave Hansen wrote:
>> On 2/8/21 8:16 AM, Jing Liu wrote:
>>> -#define XSTATE_COMPACTION_ENABLED (1ULL << 63)
>>> -
>>>   static void fill_xsave(u8 *dest, struct kvm_vcpu *vcpu)
>>>   {
>>>   	struct xregs_state *xsave = &vcpu->arch.guest_fpu->state.xsave;
>>> @@ -4494,7 +4492,8 @@ static void load_xsave(struct kvm_vcpu *vcpu, u8 *src)
>>>   	/* Set XSTATE_BV and possibly XCOMP_BV.  */
>>>   	xsave->header.xfeatures = xstate_bv;
>>>   	if (boot_cpu_has(X86_FEATURE_XSAVES))
>>> -		xsave->header.xcomp_bv = host_xcr0 | XSTATE_COMPACTION_ENABLED;
>>> +		xsave->header.xcomp_bv = XCOMP_BV_COMPACTED_FORMAT |
>>> +					 xfeatures_mask_all;
> This is wrong, xfeatures_mask_all also tracks supervisor states.
When looking at SDM Vol2 XSAVES instruction Operation part, it says as 
follows,

RFBM ← (XCR0 OR IA32_XSS) AND EDX:EAX;
COMPMASK ← RFBM OR 80000000_00000000H;
...

XCOMP_BV field in XSAVE header ← COMPMASK;


So it seems xcomp_bv also tracks supervisor states?

BRs,
Jing
>
>> Are 'host_xcr0' and 'xfeatures_mask_all' really interchangeable?  If so,
>> shouldn't we just remove 'host_xcr0' everywhere?
> I think so?  But use xfeatures_mask_user().
>
> In theory, host_xss can also be replaced with the _supervisor() and _dynamic()
> variants.  That code needs a good hard look at the _dynamic() features, which is
> currently just architectural LBRs.  E.g. I wouldn't be surprised if KVM currently
> fails to save/restore arch LBRs due to the bit not being set in host_xss.