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Re: [PATCH v2 3/3] x86/cpu-policy: Derive RSBA/RRSBA for guest policies
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Fri, 2 Jun 2023 16:38:47 +0100
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Fri, 02 Jun 2023 15:39:17 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 02/06/2023 4:29 pm, Andrew Cooper wrote:
> On 02/06/2023 11:20 am, Jan Beulich wrote:
>> On 01.06.2023 16:48, Andrew Cooper wrote:
>> What about a tool stack request leading to us setting the 2nd of the two
>> bits here, while the other was already set? IOW wouldn't we better clear
>> the other bit explicitly? (Due to the EIBRS dependency or RRSBA I think
>> this can really only happen when the tool stack requests RSBA+EIBRS, as
>> the deep deps clearing doesn't know the concept of "negative"
>> dependencies.)
> Hmm - I think there is a bug here, but it's not this simple. I think
> the only reasonable thing we can do is start rejecting bad input because
> I don't think Xen can fix up safely.
>
> Xen must not ever clear RSBA, or we've potentially made the VM unsafe
> behind the toolstack's back.
>
> If EIBRS != RRSBA, the toolstack has made a mistake. Equally too for
> RSBA && EIBRS.
>
> I think this is going to take more coffee to solve...
Actually, no.
I'm going to delete the hunk modifying recalculate_cpuid(), and move
this patch back to the meaning it had in v1 which is just "get the
policies looking correct".
It's still not supported for the toolstack to request ARCH_CAPS (the "a"
marking), and the safely logic for that can come in a subsequent series
along with the unit(ish) testing I was already planning to do.
~Andrew
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