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Subject: Re: [PATCH 0/4] Add support for XMM fast hypercalls
To:     Siddharth Chandrasekaran <sidcha@amazon.de>
Cc:     Wei Liu <wei.liu@kernel.org>, kys@microsoft.com,
        haiyangz@microsoft.com, sthemmin@microsoft.com, tglx@linutronix.de,
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        jmattson@google.com, joro@8bytes.org, graf@amazon.com,
        eyakovl@amazon.de, linux-hyperv@vger.kernel.org,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org
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 <20210408154006.GA32315@u366d62d47e3651.ant.amazon.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <53200f24-bd57-1509-aee2-0723aa8a3f6f@redhat.com>
Date:   Thu, 8 Apr 2021 17:48:19 +0200
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On 08/04/21 17:40, Siddharth Chandrasekaran 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.

What guest is that?

Paolo