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 10:45:22 +0800
From:   "Yang, Weijiang" <weijiang.yang@...el.com>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "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 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.

> The side effect will be that KVM guest FPUs now get guaranteed room for
> PASID as well as CET. I think I remember you mentioned that due to
> alignment requirements, there shouldn't usually be any size change
> though?

Yes, IIUC the precondition is AMX feature is enabled for guest so that the CET supervisor
state actually resides in the gap to next 64byte aligned address, so no actual size expansion.

> It might be nice to add that in the log, if I'm remembering
> correctly.

Sure, thanks!