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From:   Vitaly Kuznetsov <vkuznets@redhat.com>
To:     Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>, kvm@vger.kernel.org,
        x86@kernel.org, "K. Y. Srinivasan" <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        "Michael Kelley \(EOSG\)" <Michael.H.Kelley@microsoft.com>,
        Mohammed Gamal <mmorsy@redhat.com>,
        Cathy Avery <cavery@redhat.com>, Bandan Das <bsd@redhat.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 7/7] x86/kvm: use Enlightened VMCS when running on Hyper-V
References: <20180309140249.2840-1-vkuznets@redhat.com>
        <20180309140249.2840-8-vkuznets@redhat.com>
        <2747cc75-b549-61bb-9c1b-0f554a49b536@redhat.com>
        <87zi399xih.fsf@vitty.brq.redhat.com> <20180315170202.GA5180@flask>
        <20180315172852.GA21667@flask>
Date:   Thu, 15 Mar 2018 19:04:35 +0100
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 \=\?utf-8\?B\?Iidz\?\= message of "Thu,
        15 Mar 2018 18:28:52 +0100")
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Radim Krčmář <rkrcmar@redhat.com> writes:

> 2018-03-15 18:02+0100, Radim Krčmář:
>> We actually already have mov in the assembly:
>> 
>>   "cmp %%" _ASM_SP ", %c[host_rsp](%0) \n\t"
>
> Oh hell, I didn't pay attention to this line before.
>

This is still going to work if we conditionally replace it with pointer
to evmcs as you suggested before but ...


>>   "je 1f \n\t"
>>   "mov %%" _ASM_SP ", %c[host_rsp](%0) \n\t" // here
>>   __ex(ASM_VMX_VMWRITE_RSP_RDX) "\n\t"
>
> I bet this path is executed only once in VM's lifetime and what we're
> doing is wasting more resources than we're ever going to save ...
>

yes, we're not gonna save anything...

-- 
  Vitaly