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Date:   Thu, 8 Apr 2021 15:28:17 +0000
From:   Wei Liu <wei.liu@kernel.org>
To:     Siddharth Chandrasekaran <sidcha@amazon.de>
Cc:     kys@microsoft.com, haiyangz@microsoft.com, sthemmin@microsoft.com,
        wei.liu@kernel.org, tglx@linutronix.de, mingo@redhat.com,
        bp@alien8.de, x86@kernel.org, hpa@zytor.com, pbonzini@redhat.com,
        seanjc@google.com, vkuznets@redhat.com, wanpengli@tencent.com,
        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
Subject: Re: [PATCH 0/4] Add support for XMM fast hypercalls
Message-ID: <20210408152817.k4d4hjdqu7hsjllo@liuwe-devbox-debian-v2>
References: <20210407212926.3016-1-sidcha@amazon.de>
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On Wed, Apr 07, 2021 at 11:29:26PM +0200, Siddharth Chandrasekaran wrote:
> Hyper-V supports the use of XMM registers to perform fast hypercalls.
> This allows guests to take advantage of the improved performance of the
> fast hypercall interface even though a hypercall may require more than
> (the current maximum of) two general purpose registers.
> 
> The XMM fast hypercall interface uses an additional six XMM registers
> (XMM0 to XMM5) to allow the caller to pass an input parameter block of
> up to 112 bytes. Hyper-V can also return data back to the guest in the
> remaining XMM registers that are not used by the current hypercall.
> 
> 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.

Wei.