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Re: [PATCH 1/3] x86/msr: Split MSR_SPEC_CTRL handling
- To: Andrew Cooper <amc96@xxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Mon, 17 Jan 2022 12:54:54 +0100
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Mon, 17 Jan 2022 11:55:12 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 17.01.2022 12:24, Andrew Cooper wrote:
> On 17/01/2022 11:07, Jan Beulich wrote:
>> On 13.01.2022 17:38, Andrew Cooper wrote:
>>> In order to fix a VT-x bug, and support MSR_SPEC_CTRL on AMD, there will
>>> need
>>> to be three different access methods for where the guest's value lives.
>>> However, it would be better not to duplicate the #GP checking logic.
>>>
>>> guest_{rd,wr}msr() are always called first in the PV and HVM MSR paths, so
>>> we
>>> can repurpose X86EMUL_UNHANDLEABLE slightly for this. This is going to be a
>>> common pattern for other MSRs too in the future.
>> I consider this repurposing risky. Did you consider using e.g.
>> X86EMUL_DONE or X86EMUL_RETRY instead? Neither of the two is
>> presently used by the MSR machinery afaics.
>
> RETRY is used for the MSRs which can cause a p2m allocation and hit the
> paging path. DONE is not remotely appropriate for this purpose.
Well, okay then. I would have said DONE is as (in)appropriate as
UNHANDLEABLE here.
Jan
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