linux-kernel - Re: [RFC 32/33] KVM: x86: hyper-v: Implement HVCALL_TRANSLATE_VIRTUAL

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Date:   Wed, 8 Nov 2023 13:44:43 +0000
From:   Nicolas Saenz Julienne <nsaenz@...zon.com>
To:     Alexander Graf <graf@...zon.com>, <kvm@...r.kernel.org>
CC:     <linux-kernel@...r.kernel.org>, <linux-hyperv@...r.kernel.org>,
        <pbonzini@...hat.com>, <seanjc@...gle.com>, <vkuznets@...hat.com>,
        <anelkz@...zon.com>, <dwmw@...zon.co.uk>, <jgowans@...zon.com>,
        <corbert@....net>, <kys@...rosoft.com>, <haiyangz@...rosoft.com>,
        <decui@...rosoft.com>, <x86@...nel.org>,
        <linux-doc@...r.kernel.org>
Subject: Re: [RFC 32/33] KVM: x86: hyper-v: Implement
 HVCALL_TRANSLATE_VIRTUAL_ADDRESS

On Wed Nov 8, 2023 at 12:49 PM UTC, Alexander Graf wrote:
>
> On 08.11.23 12:18, Nicolas Saenz Julienne wrote:
> > Introduce HVCALL_TRANSLATE_VIRTUAL_ADDRESS, the hypercall receives a
> > GVA, generally from a less privileged VTL, and returns the GPA backing
> > it. The GVA -> GPA conversion is done by walking the target VTL's vCPU
> > MMU.
> >
> > NOTE: The hypercall implementation is incomplete and only shared for
> > completion. Additionally we'd like to move the VTL aware parts to
> > user-space.
>
>
> Yes, please :). We should handle the complete hypercall in user space if 
> possible. If you're afraid that gva -> gpa conversion may run out of 
> sync between a user space and the kvm implementations, let's introduce 
> an ioctl that allows you to perform that conversion.

I'll look into introducing a generic API that performs MMU walks. The
devil is in the details though, the hypercall introduces flags like:

• HV_TRANSLATE_GVA_TLB_FLUSH_INHIBIT: Indicates that the TlbFlushInhibit
  flag in the virtual processor’s HvRegisterInterceptSuspend register
  should be set as a consequence of a successful return. This prevents
  other virtual processors associated with the target partition from
  flushing the stage 1 TLB of the specified virtual processor until
  after the TlbFlushInhibit flag is cleared.

Which make things trickier.

Nicolas