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From:   Thomas Gleixner <tglx@linutronix.de>
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To:     Paolo Bonzini <pbonzini@redhat.com>,
        "Liu, Jing2" <jing2.liu@intel.com>,
        LKML <linux-kernel@vger.kernel.org>
Cc:     "x86@kernel.org" <x86@kernel.org>,
        "Bae, Chang Seok" <chang.seok.bae@intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Arjan van de Ven <arjan@linux.intel.com>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "Nakajima, Jun" <jun.nakajima@intel.com>,
        Jing Liu <jing2.liu@linux.intel.com>,
        "seanjc@google.com" <seanjc@google.com>,
        "Cooper, Andrew" <andrew.cooper3@citrix.com>
Subject: Re: [patch 13/31] x86/fpu: Move KVMs FPU swapping to FPU core
In-Reply-To: <87wnmf66m5.ffs@tglx>
References: <871r4p9fyh.ffs@tglx>
 <ec9c761d-4b5c-71e2-c1fc-d256b6b78c04@redhat.com>
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 <6bbc5184-a675-1937-eb98-639906a9cf15@redhat.com> <87wnmf66m5.ffs@tglx>
Date:   Thu, 14 Oct 2021 16:37:07 +0200
Message-ID: <87o87r65bg.ffs@tglx>
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On Thu, Oct 14 2021 at 16:09, Thomas Gleixner wrote:
> On Thu, Oct 14 2021 at 11:01, Paolo Bonzini wrote:
>
> Also you really should not wait until _all_ dynamic states are cleared
> in guest XFD. Because a guest which has bit 18 and 19 available but only
> uses one of them is going to trap on every other context switch due to
> XFD writes.
>
> So you check for
>
>    (guest_xfd & guest_perm) != guest_perm)
>
> and
>
>    (guest_xr0 & guest_perm) != 0
>
> If both are true, then you reallocate the buffers for _all_ permitted
> states _and_ set XFD to pass through.

And for that to work we must write XFD _before_ XSETBV in the guest boot
phase.

Thanks,

        tglx