Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.37 as permitted sender) client-ip=23.128.96.37;
From:   Paul Durrant <xadimgnik@gmail.com>
Message-ID: <94697586-7600-420d-a91b-2829019dab7c@xen.org>
Date:   Wed, 22 Nov 2023 10:55:30 +0000
MIME-Version: 1.0
User-Agent: Mozilla Thunderbird
Reply-To: paul@xen.org
Subject: Re: [PATCH v8 15/15] KVM: xen: allow vcpu_info content to be 'safely'
 copied
Content-Language: en-US
To:     David Woodhouse <dwmw2@infradead.org>,
        Sean Christopherson <seanjc@google.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <20231121180223.12484-1-paul@xen.org>
 <20231121180223.12484-16-paul@xen.org>
 <4a76b7dc9055485d9e2592b395e60221dc349abf.camel@infradead.org>
 <7c7238a9c8b0dc6bc865407ba804a651cdfdb044.camel@infradead.org>
Organization: Xen Project
In-Reply-To: <7c7238a9c8b0dc6bc865407ba804a651cdfdb044.camel@infradead.org>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk

On 22/11/2023 10:39, David Woodhouse wrote:
> On Tue, 2023-11-21 at 22:53 +0000, David Woodhouse wrote:
>> On Tue, 2023-11-21 at 18:02 +0000, Paul Durrant wrote:
>>> From: Paul Durrant <pdurrant@amazon.com>
>>>
>>> If the guest sets an explicit vcpu_info GPA then, for any of the first 32
>>> vCPUs, the content of the default vcpu_info in the shared_info page must be
>>> copied into the new location. Because this copy may race with event
>>> delivery (which updates the 'evtchn_pending_sel' field in vcpu_info) there
>>> needs to be a way to defer that until the copy is complete.
>>> Happily there is already a shadow of 'evtchn_pending_sel' in kvm_vcpu_xen
>>> that is used in atomic context if the vcpu_info PFN cache has been
>>> invalidated so that the update of vcpu_info can be deferred until the
>>> cache can be refreshed (on vCPU thread's the way back into guest context).
>>>
>>> Also use this shadow if the vcpu_info cache has been *deactivated*, so that
>>> the VMM can safely copy the vcpu_info content and then re-activate the
>>> cache with the new GPA. To do this, stop considering an inactive vcpu_info
>>> cache as a hard error in kvm_xen_set_evtchn_fast().
>>>
>>> Signed-off-by: Paul Durrant <pdurrant@amazon.com>
>>> Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
>>
>> Wait, didn't we realise that this leaves the bits set in the shadow
>> evtchn_pending_sel that get lost on migration?
>>

Indeed we did not, but that's not something that *this* patch, or even 
this series, is dealing with.  We also know that setting the 'width' of 
shared_info has some issues, but again, can we keep that for other 
patches? The series is at v9 and has already suffered a fair amount of 
scope-creep.

   Paul