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

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Date:   Fri, 9 Apr 2021 09:59:17 +0200
From:   Siddharth Chandrasekaran <sidcha@...zon.de>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
CC:     Paolo Bonzini <pbonzini@...hat.com>, <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>,
        <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>, Wei Liu <wei.liu@...nel.org>
Subject: Re: [PATCH 0/4] Add support for XMM fast hypercalls

On Fri, Apr 09, 2021 at 09:42:41AM +0200, Vitaly Kuznetsov wrote:
> Siddharth Chandrasekaran <sidcha@...zon.de> writes:
> > On Thu, Apr 08, 2021 at 04:30:18PM +0000, Wei Liu wrote:
> >> On Thu, Apr 08, 2021 at 05:54:43PM +0200, Siddharth Chandrasekaran wrote:
> >> > On Thu, Apr 08, 2021 at 05:48:19PM +0200, Paolo Bonzini wrote:
> >> > > 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?
> >> >
> >> > It is a Windows Server 2016.
> >>
> >> Can you be more specific? Are you implementing some hypercalls from
> >> TLFS? If so, which ones?
> >
> > Yes all of them are from TLFS. We are implementing VSM and there are a
> > bunch of hypercalls that we have implemented to manage VTL switches,
> > memory protection and virtual interrupts.
> 
> Wow, sounds awesome! Do you plan to upstream this work?

Yes, that is the plan :-)

> > The following 3 hypercalls that use the XMM fast hypercalls are relevant
> > to this patch set:
> >
> > HvCallModifyVtlProtectionMask
> > HvGetVpRegisters
> > HvSetVpRegisters
> 
> It seems AccessVSM and AccessVpRegisters privilges have implicit
> dependency on XMM input/output. This will need to be enforced in KVM
> userspace.

Noted.

~ Sid.



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