linux-kernel - Re: [PATCH RFC 3/7] kvm: x86: XSAVE state and XFD MSRs context switch

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Date:   Mon, 22 Feb 2021 16:51:46 +0800
From:   "Liu, Jing2" <jing2.liu@...ux.intel.com>
To:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, jing2.liu@...el.com
Subject: Re: [PATCH RFC 3/7] kvm: x86: XSAVE state and XFD MSRs context switch



On 2/9/2021 2:55 AM, Konrad Rzeszutek Wilk wrote:
> On Mon, Feb 08, 2021 at 07:12:22PM +0100, Paolo Bonzini wrote:
[...]
>>
>> However, running the host with _more_ bits set than necessary in XFD should
>> not be a problem as long as the host doesn't use the AMX instructions.  So
>> perhaps Jing can look into keeping XFD=0 for as little time as possible, and
>> XFD=host_XFD|guest_XFD as much as possible.
> This sounds like the lazy-fpu (eagerfpu?) that used to be part of the
> kernel? I recall that we had a CVE for that - so it may also be worth
> double-checking that we don't reintroduce that one.
Not sure if this means lazy restore, but the spec mentions that
"System software should not use XFD to implement a 'lazy restore' approach
to management of the XTILEDATA state component." One reason is XSAVE(S)
will lose the xTILEDATA when XFD[i] is nonzero.

Thanks,
Jing