linux-kernel - Re: [PATCH 0/4] Add support for XMM fast hypercalls

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Message-ID: <20210408154006.GA32315@u366d62d47e3651.ant.amazon.com>
Date:   Thu, 8 Apr 2021 17:40:07 +0200
From:   Siddharth Chandrasekaran <sidcha@...zon.de>
To:     Paolo Bonzini <pbonzini@...hat.com>
CC:     Wei Liu <wei.liu@...nel.org>, <kys@...rosoft.com>,
        <haiyangz@...rosoft.com>, <sthemmin@...rosoft.com>,
        <tglx@...utronix.de>, <mingo@...hat.com>, <bp@...en8.de>,
        <x86@...nel.org>, <hpa@...or.com>, <seanjc@...gle.com>,
        <vkuznets@...hat.com>, <wanpengli@...cent.com>,
        <jmattson@...gle.com>, <joro@...tes.org>, <graf@...zon.com>,
        <eyakovl@...zon.de>, <linux-hyperv@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <kvm@...r.kernel.org>
Subject: Re: [PATCH 0/4] Add support for XMM fast hypercalls

On Thu, Apr 08, 2021 at 05:30:26PM +0200, Paolo Bonzini wrote:
> CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you can confirm the sender and know the content is safe.
>
>
>
> On 08/04/21 17:28, Wei Liu wrote:
> > > Although the Hyper-v TLFS mentions that a guest cannot use this feature
> > > unless the hypervisor advertises support for it, some hypercalls which
> > > we plan on upstreaming in future uses them anyway.
> >
> > No, please don't do this. Check the feature bit(s) before you issue
> > hypercalls which rely on the extended interface.
>
> Perhaps Siddharth should clarify this, but I read it as Hyper-V being
> buggy and using XMM arguments unconditionally.

The guest is at fault here as it expects Hyper-V to consume arguments
from XMM registers for certain hypercalls (that we are working) even if
we didn't expose the feature via CPUID bits.

~ Sid.



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