linux-kernel - Re: [PATCH v6 02/25] x86/fpu/xstate: Fix guest fpstate allocation size calculation

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Date:   Fri, 15 Sep 2023 16:35:46 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Yang, Weijiang" <weijiang.yang@...el.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     "peterz@...radead.org" <peterz@...radead.org>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "Gao, Chao" <chao.gao@...el.com>,
        "john.allen@....com" <john.allen@....com>
Subject: Re: [PATCH v6 02/25] x86/fpu/xstate: Fix guest fpstate allocation
 size calculation

On Fri, 2023-09-15 at 10:45 +0800, Yang, Weijiang wrote:
> On 9/15/2023 6:45 AM, Edgecombe, Rick P wrote:
> > On Thu, 2023-09-14 at 02:33 -0400, Yang Weijiang wrote:
> > > Fix guest xsave area allocation size from
> > > fpu_user_cfg.default_size
> > > to
> > > fpu_kernel_cfg.default_size so that the xsave area size is
> > > consistent
> > > with fpstate->size set in __fpstate_reset().
> > > 
> > > With the fix, guest fpstate size is sufficient for KVM supported
> > > guest
> > > xfeatures.
> > > 
> > > Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
> > There is no fix (Fixes: ...) here, right?
> 
> Ooh, I got it lost during rebase, thanks!
> 
> > I think this change is needed
> > to make sure KVM guests can support supervisor features. But KVM
> > CET
> > support (to follow in future patches) will be the first one, right?
> 
> Exactly, the existing code takes only user xfeatures into account,
> and we have more
> and more CPU features rely on supervisor xstates.

If KVM is not using any supervisor features, then pre CET KVM support I
think the current code is more correct.