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Re: [PATCH v2 2/6] x86/HVM: restrict guest-induced WBINVD to cache writeback
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Tue, 13 May 2025 15:24:31 +0200
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- Cc: Wei Liu <wl@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Tue, 13 May 2025 13:24:56 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 13.05.2025 15:12, Andrew Cooper wrote:
> On 03/05/2023 10:45 am, Jan Beulich wrote:
>> We allow its use for writeback purposes only anyway, so let's also carry
>> these out that way on capable hardware.
By implication of what you say ...
>> With it now known that WBNOINVD uses the same VM exit code as WBINVD for
>> both SVM and VT-x, we can now also expose the feature that way without
>> further distinguishing the specific cases of those VM exits. Note that
>> on SVM this builds upon INSTR_WBINVD also covering WBNOINVD, as the
>> decoder won't set prefix related bits for this encoding in the resulting
>> canonicalized opcode.
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>
> I realise this is an old series,
Well, it's old because no-one cared to look at the v2 posting in over 2
years.
> but this patch is unsafe with AMD SEV-SNP, and with CLX devices.
>
> Both have cases needing "genuinely evicted from the cache" semantics.
... here, are you suggesting that our present behavior is wrong? We don't
have SEV-SNP support yet, so there it would only be "latently wrong" (and
hence the change here would still be correct for now). What CLX is in
this context I'm afraid I don't know (since surely you're not talking
about vapes, the closest I can spot in search results is something gaming
related, yet still not becoming very clear). I might have guessed
Cascade Lake X, but then why would just that one model be a problem?
Jan
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