linux-kernel - Re: [PATCH 03/16] KVM: x86: set gfn-to-pfn cache length consistently with VM word size

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Date:   Sun, 13 Nov 2022 13:32:46 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org, mhal@...x.co,
        Paul Durrant <pdurrant@...zon.co.uk>,
        Metin Kaya <metikaya@...zon.co.uk>
Subject: Re: [PATCH 03/16] KVM: x86: set gfn-to-pfn cache length
 consistently with VM word size

On Fri, 2022-10-28 at 13:36 +0200, Paolo Bonzini wrote:
> We _should_ be following the Xen API, which does not even say that the
> areas have to fit in a single page.  In fact, even Linux's
> 
>          struct vcpu_register_runstate_memory_area area;
> 
>          area.addr.v = &per_cpu(xen_runstate, cpu);
>          if (HYPERVISOR_vcpu_op(VCPUOP_register_runstate_memory_area,
>                                 xen_vcpu_nr(cpu), &area))
> 
> could fail or not just depending on the linker's whims, if I'm not
> very confused.
> 
> Other data structures *do* have to fit in a page, but the runstate area
> does not and it's exactly the one where the cache comes the most handy.
> For this I'm going to wait for David to answer.

Yeah, I recall vetting a bunch of these to ensure that it's safe to
assume that it does fit within the page... but that clearly isn't true
for the runstate_area.

As things stand, I believe a guest is perfectly entitled to provide a
region which crosses a page boundary, and Xen copes with that. But as
you say, KVM doesn't.

However, I don't think this *is* the case where the cache comes in the
most handy. The cache is really useful where we have to do *atomic*
operations on guest addresses, and doing so directly is a whole lot
nicer than futex-style try-it-and-fail-gracefully operations on
userspace addresses.

For the runstate area, I think we can live with using a gfn_to_hva
cache instead, and writing via the userspace address (with appropriate
atomicity for the RUNSTATE_runnable case as we have at the moment
gating the refresh).

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