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    SubjectRe: [PATCH] x86: also disable FSRM if ERMS is disabled
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    On 07. 10. 22, 20:08, Daniel Verkamp wrote:
    > On Fri, Sep 23, 2022 at 10:51 AM Borislav Petkov <bp@alien8.de> wrote:
    >>
    >> On Fri, Sep 23, 2022 at 10:25:05AM -0700, Daniel Verkamp wrote:
    >>> Yes, we hit this in crosvm when booting the guest kernel with either
    >>> OVMF or u-boot on an Intel 12th Gen CPU. The guest kernel boots fine
    >>> when loaded directly (using the crosvm kernel loader and not running
    >>> any firmware setup in the guest), but it crashes when booting with
    >>> firmware inside the first forward memmove() after alternatives are set
    >>> up (which happens to be in printk). I haven't gotten to the bottom of
    >>> why exactly using firmware is causing this to be set up in an
    >>> inconsistent way, but this is a real-world situation, not just a
    >>> hypothetical.
    >>
    >> Sounds like broken virt firmware or so. And if that is not an issue on
    >> baremetal, then the virt stack should be fixed - not the kernel.
    >>
    >>> Now that I look at it with fresh eyes again, maybe we should instead
    >>> directly patch the memmove FSRM alternative so that the flag-set
    >>> version just does the same jmp as the ERMS one. I can prepare a patch
    >>> for that instead of (or in addition to) this one if that sounds
    >>> better.
    >>
    >> So, if the virt firmware deviates from how the real hardware behaves,
    >> then the kernel needs no fixing.
    >>
    >> So you'd have to figure out why is the virt firmware causing this and
    >> not baremetal.
    >>
    >> Then we can talk about fixes.
    >
    > Hi Borislav,
    >
    > We found that the IA32_MISC_ENABLE MSR setup was missing in the crosvm
    > firmware boot path (but not when directly booting a kernel, which is
    > why it did not get noticed for a while). Setting the fast string bit
    > in the MSR avoids the issue.
    >
    > However, I still think it would be appropriate to apply this patch or
    > something like it, since there could be a CPU, microcode update, BIOS,
    > etc. that clears this bit while still having the CPUID flags for FSRM
    > and ERMS.

    Let me resurrect this thread... Our customer has an AMD CPU which has
    indeed both capabilities under normal circumstances. But they have a
    cool UEFI BIOS too. They say:

    """
    In AMD platform, while disalbe ERMS(Enhanced Rep MOVSB/STOSB) in UEFI
    (system setup -> processor -> Enhanced Rep MOVSB/STOSB), the OS can't
    boot normally.
    """

    That is exactly the case here. So can we have the patch (the original
    one, the one below or a better one) to fix this?

    > The Intel SDM says: "Software can disable fast-string
    > operation by clearing the fast-string-enable bit (bit 0) of
    > IA32_MISC_ENABLE MSR", so it's not an invalid configuration for this
    > bit to be unset.
    >
    > Additionally, something like this avoids the problem by making the
    > FSRM case jump directly to the REP MOVSB rather than falling through
    > to the ERMS jump in the next instruction, which seems like basically
    > free insurance (but if the FSRM flag gets used somewhere else in the
    > future, having it set consistently with ERMS is probably still a good
    > idea, per the original patch):
    >
    > diff --git a/arch/x86/lib/memmove_64.S b/arch/x86/lib/memmove_64.S
    > index 724bbf83eb5b..8ac557409c7d 100644
    > --- a/arch/x86/lib/memmove_64.S
    > +++ b/arch/x86/lib/memmove_64.S
    > @@ -38,7 +38,7 @@ SYM_FUNC_START(__memmove)
    >
    > /* FSRM implies ERMS => no length checks, do the copy directly */
    > .Lmemmove_begin_forward:
    > - ALTERNATIVE "cmp $0x20, %rdx; jb 1f", "", X86_FEATURE_FSRM
    > + ALTERNATIVE "cmp $0x20, %rdx; jb 1f", "jmp .Lmemmove_erms",
    > X86_FEATURE_FSRM
    > ALTERNATIVE "", "jmp .Lmemmove_erms", X86_FEATURE_ERMS
    >
    > And hey, this means one less instruction to execute in the FSRM path. :)

    thanks,
    --
    js
    suse labs

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