From: Xiaoyao Li <xiaoyao.li@intel.com>
To: Gerd Hoffmann <kraxel@redhat.com>
Cc: qemu-devel@nongnu.org, kvm@vger.kernel.org,
Marcelo Tosatti <mtosatti@redhat.com>,
Marcel Apfelbaum <marcel.apfelbaum@gmail.com>,
Eduardo Habkost <eduardo@habkost.net>,
Paolo Bonzini <pbonzini@redhat.com>,
Richard Henderson <richard.henderson@linaro.org>,
"Michael S. Tsirkin" <mst@redhat.com>,
Sergio Lopez <slp@redhat.com>
Subject: Re: [PATCH 0/2] expose host-phys-bits to guest
Date: Fri, 2 Sep 2022 08:10:00 +0800 [thread overview]
Message-ID: <34be4132-53f4-8779-1ada-68aa554e0eac@intel.com> (raw)
In-Reply-To: <20220901161741.dadmguwv25sk4h6i@sirius.home.kraxel.org>
On 9/2/2022 12:17 AM, Gerd Hoffmann wrote:
> On Thu, Sep 01, 2022 at 10:36:19PM +0800, Xiaoyao Li wrote:
>> On 9/1/2022 9:58 PM, Gerd Hoffmann wrote:
>>
>>>> Anyway, IMO, guest including guest firmware, should always consult from
>>>> CPUID leaf 0x80000008 for physical address length.
>>>
>>> It simply can't for the reason outlined above. Even if we fix qemu
>>> today that doesn't solve the problem for the firmware because we want
>>> backward compatibility with older qemu versions. Thats why I want the
>>> extra bit which essentially says "CPUID leaf 0x80000008 actually works".
>>
>> I don't understand how it backward compatible with older qemu version. Old
>> QEMU won't set the extra bit you introduced in this series, and all the
>> guest created with old QEMU will become untrusted on CPUID leaf 0x80000008 ?
>
> Correct, on old qemu firmware will not trust CPUID leaf 0x80000008.
> That is not worse than the situation we have today, currently the
> firmware never trusts CPUID leaf 0x80000008.
>
> So the patches will improves the situation for new qemu only, but I
> don't see a way around that.
>
I see.
But IMHO, I don't think it's good that guest firmware workaround the
issue on its own. Instead, it's better to just trust CPUID leaf
0x80000008 and fail if the given physical address length cannot be
virtualized/supported.
It's just the bug of VMM to virtualize the physical address length. The
correction direction is to fix the bug not the workaround to hide the bug.
next prev parent reply other threads:[~2022-09-02 0:10 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-08-31 12:50 [PATCH 0/2] expose host-phys-bits to guest Gerd Hoffmann
2022-08-31 12:50 ` [PATCH 1/2] [hack] reserve bit KVM_HINTS_HOST_PHYS_BITS Gerd Hoffmann
2022-08-31 13:05 ` Michael S. Tsirkin
2022-08-31 12:50 ` [PATCH 2/2] [RfC] expose host-phys-bits to guest Gerd Hoffmann
2022-08-31 12:58 ` Claudio Fontana
2022-09-01 6:07 ` [PATCH 0/2] " Xiaoyao Li
2022-09-01 13:58 ` Gerd Hoffmann
2022-09-01 14:36 ` Xiaoyao Li
2022-09-01 16:17 ` Gerd Hoffmann
2022-09-02 0:10 ` Xiaoyao Li [this message]
2022-09-02 6:07 ` Gerd Hoffmann
2022-09-02 6:35 ` Michael S. Tsirkin
2022-09-02 8:44 ` Gerd Hoffmann
2022-09-04 20:37 ` Michael S. Tsirkin
2022-09-05 7:39 ` Gerd Hoffmann
2022-09-01 14:55 ` Claudio Fontana
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