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From:   Vitaly Kuznetsov <vkuznets@redhat.com>
To:     Paolo Bonzini <pbonzini@redhat.com>,
        Siddharth Chandrasekaran <sidcha@amazon.de>
Cc:     Alexander Graf <graf@amazon.com>,
        Evgeny Iakovlev <eyakovl@amazon.de>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Sean Christopherson <seanjc@google.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH 3/4] KVM: x86: kvm_hv_flush_tlb use inputs from XMM
 registers
In-Reply-To: <01fc0ac9-f159-d3df-6c8c-8f8122fe31ea@redhat.com>
References: <20210407211954.32755-1-sidcha@amazon.de>
 <20210407211954.32755-4-sidcha@amazon.de>
 <87eefl7zp4.fsf@vitty.brq.redhat.com>
 <01fc0ac9-f159-d3df-6c8c-8f8122fe31ea@redhat.com>
Date:   Thu, 08 Apr 2021 14:52:16 +0200
Message-ID: <878s5t7xbz.fsf@vitty.brq.redhat.com>
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Paolo Bonzini <pbonzini@redhat.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