Messages in this thread | | | Date | Wed, 22 Feb 2023 22:17:35 +0000 | From | Andrew Cooper <> | Subject | Re: x86: AMD Zen2 ymm registers rolling back |
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On 22/02/2023 9:26 pm, Tavis Ormandy wrote: > On Wed, Feb 22, 2023 at 11:09:51AM +0100, Borislav Petkov wrote: >> On Wed, Feb 22, 2023 at 09:38:09AM +0000, Andrew Cooper wrote: >>> This sounds suspiciously like an errata which was fixed with a ucode >>> update last year. >> Yes, it looks like it. >> >> Alternatively, you can try booting with "clearcpuid=xsaves" - that >> should take care of your observation too but yeah, you should rather >> update your microcode. >> > Thanks - confirmed, it *doesn't* repro with 0x8301055, but does repro > with 0x830104d.
For completeness, this is erratum 1386, details of which can be found in https://www.amd.com/system/files/TechDocs/56323-PUB_1.00.pdf amongst others.
That said, looking at the list of preconditions to tickle this erratum, I don't see anything in your code which would cause a change in MXCSR. Which most likely means there's a path in the kernel modifying MXCSR and that doesn't sound like a thing that ought to be happening by default.
> Annoyingly, I thought I was using the most recent microcode, but it seems > like there is some bug and debian wasn't applying it at boot. > > That seems like a scary errata :-/
Honestly, this is tame compared to some of the things which get fixed in ucode...
~Andrew
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