linux-kernel - Re: [PATCH v3 7/7] x86/kvm: use Enlightened VMCS when running on Hyper-V

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Message-ID: <87sh919pvw.fsf@vitty.brq.redhat.com>
Date:   Thu, 15 Mar 2018 19:04:35 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Radim Krčmář <rkrcmar@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        x86@...nel.org, "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "Michael Kelley \(EOSG\)" <Michael.H.Kelley@...rosoft.com>,
        Mohammed Gamal <mmorsy@...hat.com>,
        Cathy Avery <cavery@...hat.com>, Bandan Das <bsd@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 7/7] x86/kvm: use Enlightened VMCS when running on Hyper-V

Radim Krčmář <rkrcmar@...hat.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