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

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Date:   Thu, 8 Apr 2021 15:28:17 +0000
From:   Wei Liu <wei.liu@...nel.org>
To:     Siddharth Chandrasekaran <sidcha@...zon.de>
Cc:     kys@...rosoft.com, haiyangz@...rosoft.com, sthemmin@...rosoft.com,
        wei.liu@...nel.org, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, x86@...nel.org, hpa@...or.com, pbonzini@...hat.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 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.