linux-kernel - Re: [PATCH 3/4] KVM: x86: kvm_hv_flush

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Message-ID: <878s5t7xbz.fsf@vitty.brq.redhat.com>
Date:   Thu, 08 Apr 2021 14:52:16 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Siddharth Chandrasekaran <sidcha@...zon.de>
Cc:     Alexander Graf <graf@...zon.com>,
        Evgeny Iakovlev <eyakovl@...zon.de>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Sean Christopherson <seanjc@...gle.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>
Subject: Re: [PATCH 3/4] KVM: x86: kvm_hv_flush_tlb use inputs from XMM
 registers

Paolo Bonzini <pbonzini@...hat.com> writes:

> On 08/04/21 14:01, Vitaly Kuznetsov wrote:
>> 
>> Also, we can probably defer kvm_hv_hypercall_read_xmm() until we know
>> how many regs we actually need to not read them all (we will always
>> need xmm[0] I guess so we can as well read it here).
>
> The cost is get/put FPU, so I think there's not much to gain from that.
>

Maybe, I just think that in most cases we will only need xmm0. To make
the optimization work we can probably do kvm_get_fpu() once we figured
out that we're dealing with XMM hypercall and do kvm_put_fpu() when
we're done processing hypercall parameters. This way we don't need to do
get/put twice. We can certainly leave this idea to the (possible) future
optimizations.

-- 
Vitaly