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Subject: Re: [PATCH RFC 03/39] KVM: x86/xen: register shared_info page
To:     Ankur Arora <ankur.a.arora@oracle.com>
Cc:     Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        David Woodhouse <dwmw2@infradead.org>
References: <20190220201609.28290-1-joao.m.martins@oracle.com>
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From:   Joao Martins <joao.m.martins@oracle.com>
Message-ID: <474f59b0-184d-12a1-4172-c65d8970810e@oracle.com>
Date:   Wed, 2 Dec 2020 10:50:31 +0000
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On 12/2/20 5:17 AM, Ankur Arora wrote:
> On 2020-12-01 5:26 p.m., David Woodhouse wrote
>> On Tue, 2020-12-01 at 16:40 -0800, Ankur Arora wrote:
>>> On 2020-12-01 5:07 a.m., David Woodhouse wrote:

[...]

>>>> If that was allowed, wouldn't it have been a much simpler fix for
>>>> CVE-2019-3016? What am I missing?
>>>
>>> Agreed.
>>>
>>> Perhaps, Paolo can chime in with why KVM never uses pinned page
>>> and always prefers to do cached mappings instead?
>>>
>>>>
>>>> Should I rework these to use kvm_write_guest_cached()?
>>>
>>> kvm_vcpu_map() would be better. The event channel logic does RMW operations
>>> on shared_info->vcpu_info.
>>
>> I've ported the shared_info/vcpu_info parts and made a test case, and
>> was going back through to make it use kvm_write_guest_cached(). But I
>> should probably plug on through the evtchn bits before I do that.
>>
>> I also don't see much locking on the cache, and can't convince myself
>> that accessing the shared_info page from multiple CPUs with
>> kvm_write_guest_cached() or kvm_map_gfn() is sane unless they each have
>> their own cache.
> 
> I think you could get a VCPU specific cache with kvm_vcpu_map().
> 

steal clock handling creates such a mapping cache (struct gfn_to_pfn_cache).

	Joao