linux-kernel - Re: [PATCH v5 07/10] KVM: xen: allow vcpu

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Message-ID: <8f61e1618f23e975f30e552c09787c5f82ee89f3.camel@infradead.org>
Date:   Fri, 22 Sep 2023 16:44:14 +0100
From:   David Woodhouse <dwmw2@...radead.org>
To:     Paul Durrant <paul@....org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     Paul Durrant <pdurrant@...zon.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org
Subject: Re: [PATCH v5 07/10] KVM: xen: allow vcpu_info to be mapped by
 fixed HVA

On Fri, 2023-09-22 at 15:00 +0000, Paul Durrant wrote:
> From: Paul Durrant <pdurrant@...zon.com>
> 
> If the guest does not explicitly set the GPA of vcpu_info structure in
> memory then, for guests with 32 vCPUs or fewer, the vcpu_info embedded
> in the shared_info page may be used. As described in a previous commit,
> the shared_info page is an overlay at a fixed HVA within the VMM, so in
> this case it also more optimal to activate the vcpu_info cache with a
> fixed HVA to avoid unnecessary invalidation if the guest memory layout
> is modified.
> 
> Signed-off-by: Paul Durrant <pdurrant@...zon.com>

But it should *only* be defined as an HVA in the case where it's the
one in the shinfo. Otherwise, it's defined by its GPA.

Which almost makes me want to see a sanity check that it precisely
equals &shinfo->vcpu_info[vcpu->arch.xen.vcpu_id].

Which brings me back around the circle again to wonder why we don't
just *default* to it.... you hate me, don't you?

Your previous set of patches did that, and it did end requiring that
the VMM restore both VCPU_INFO and VCPU_ID for each vCPU *before*
restoring the SHARED_INFO_HVA on resume, but wasn't that OK?




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