linux-kernel - Re: Hang when booting guest kernels compiled with clang after SRSO mitigations

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Message-ID: <CAKwvOdmejxO=_KBuw1WZGgh=MXNNbfWH03TpEFO47vkKWq04Cg@mail.gmail.com>
Date:   Fri, 11 Aug 2023 17:42:04 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Nathan Chancellor <nathan@...nel.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
        Borislav Petkov <bp@...en8.de>
Subject: Re: Hang when booting guest kernels compiled with clang after SRSO mitigations

On Fri, Aug 11, 2023 at 9:12 AM Borislav Petkov <bp@...en8.de> wrote:
>
> On Fri, Aug 11, 2023 at 09:03:18AM -0700, Sean Christopherson wrote:
> > Might be the flags bug that borks KVM's fastop() emulation.  If that fixes things,
> > my guess is that bringing APs out of WFS somehow triggers emulation.
>
> I was just about to connect you two guys, thanks Sean!
>
> > https://lore.kernel.org/all/20230811155255.250835-1-seanjc@google.com
>
> Nathan, if you could test, that would be cool.

Nathan confirmed on IRC (since Sean isn't there; Sean what are you
doing, you know we have corp IRCCloud accounts, yeah?):

<nathanchance>
bpetkov: Seems like Sean bailed you out :P his patch appears to fix
the issue on the Ryzen 3 box, about to test the Threadripper
bpetkov: Will have results shortly assuming the machine boots :^) is
it possible that clang is generating a code sequence that triggers
this issue that gcc does not?

<bpetkov>
yeah, something about rFLAGS gets clobbered in the clang variant while
gcc doesn't
dunno if this is a more serious code generation issue

I'm not familiar enough with the relevant code to make a call there,
but perhaps Sean has more context and can help us deduce if that's the
case?

>
> Also, Nick has another patch for -mno-shared, it probably isn't fixing
> yours but it would be good to test it too, just in case:
>
> https://github.com/ClangBuiltLinux/linux/issues/1911#issuecomment-1674993796

So far it looks like it's working for folks. Fixing that issue is the
lowest priority issue of the three we found; I'll send it formally
next week.

I literally just had my hard drive fail on my main dev box (that's two
machines fail in one week; laptop wont power on anymore; down to one
machine left).  Going to see if fsck can help at all; worst case I may
need Nathan to formally send it for me next week.  Let's see if I can
recover this machine first...what a way to end the week. SMH

>
> Thx!
>
> --
> Regards/Gruss,
>     Boris.
>
> https://people.kernel.org/tglx/notes-about-netiquette

-- 
Thanks,
~Nick Desaulniers