linux-kernel - Re: [PATCH 8/8] KVM: xen: automatically use the vcpu_info embedded in shared

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Date:   Thu, 14 Sep 2023 11:17:36 +0200
From:   Paul Durrant <xadimgnik@...il.com>
To:     David Woodhouse <dwmw2@...radead.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 8/8] KVM: xen: automatically use the vcpu_info embedded in
 shared_info

On 14/09/2023 10:09, David Woodhouse wrote:
> On Thu, 2023-09-14 at 08:49 +0000, Paul Durrant wrote:
>> From: Paul Durrant <pdurrant@...zon.com>
>>
>> Add a KVM_XEN_HVM_CONFIG_DEFAULT_VCPU_INFO flag so that the VMM knows it
>> need only set the KVM_XEN_VCPU_ATTR_TYPE_VCPU_INFO attribute in response to
>> a VCPUOP_register_vcpu_info hypercall, and modify get_vcpu_info_cache()
>> to pass back a pointer to the shared info pfn cache (and appropriate
>> offset) for any of the first 32 vCPUs if the attribute has not been set.
> 
> Might be simpler just to link this behaviour to the
> KVM_XEN_ATTR_TYPE_SHARED_INFO_HVA support? If userspace sets the
> shared_info as an HVA, then the default vcpu_info will be used therein.

Well there's no problem in using the default embedded vcpu_info even if 
the guests sets the shared_info via GPA... it still saves the VMM making 
a few ioctls. So do we really want to link the two things?

   Paul