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Re: [PATCH] x86/cpuid: Clobber CPUID leaves 0x800000{1d..20}
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 7 Apr 2022 16:27:11 +0200
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Thu, 07 Apr 2022 14:27:39 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 07.04.2022 03:01, Andrew Cooper wrote:
> c/s 1a914256dca5 increased the AMD max leaf from 0x8000001c to 0x80000021, but
> did not adjust anything in the calculate_*_policy() chain. As a result, on
> hardware supporting these leaves, we read the real hardware values into the
> raw policy, then copy into host, and all the way into the PV/HVM default
> policies.
>
> All 4 of these leaves have enable bits (first two by TopoExt, next by SEV,
> next by PQOS), so any software following the rules is fine and will leave them
> alone. However, leaf 0x8000001d takes a subleaf input and at least two
> userspace utilities have been observed to loop indefinitely under Xen (clearly
> waiting for eax to report "no more cache levels").
>
> Such userspace is buggy, but Xen's behaviour isn't great either.
Just another remark, since I stumbled across this again while preparing
the backports: I'm not convinced such user space is to be called buggy.
Generic CPUID dumping tools won't normally look for particular features.
Their knowledge is usually limited to knowing where sub-leaves exist and
how to determine how many of them there are. Anything beyond that would
make a supposedly simple tool complicated.
Jan
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