linux-kernel - Re: [PATCH] MIPS: tlbex: Fix build_restore

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Date:   Wed, 23 Oct 2019 22:02:19 -0700
From:   Paul Burton <paulburton@...nel.org>
To:     Paul Burton <paulburton@...nel.org>
CC:     linux-mips@...r.kernel.org
Subject: Re: [PATCH] MIPS: tlbex: Fix build_restore_pagemask KScratch restore

Hello,

Paul Burton wrote:
> build_restore_pagemask() will restore the value of register $1/$at when
> its restore_scratch argument is non-zero, and aims to do so by filling a
> branch delay slot. Commit 0b24cae4d535 ("MIPS: Add missing EHB in mtc0
> -> mfc0 sequence.") added an EHB instruction (Execution Hazard Barrier)
> prior to restoring $1 from a KScratch register, in order to resolve a
> hazard that can result in stale values of the KScratch register being
> observed. In particular, P-class CPUs from MIPS with out of order
> execution pipelines such as the P5600 & P6600 are affected.
> 
> Unfortunately this EHB instruction was inserted in the branch delay slot
> causing the MFC0 instruction which performs the restoration to no longer
> execute along with the branch. The result is that the $1 register isn't
> actually restored, ie. the TLB refill exception handler clobbers it -
> which is exactly the problem the EHB is meant to avoid for the P-class
> CPUs.
> 
> Similarly build_get_pgd_vmalloc() will restore the value of $1/$at when
> its mode argument equals refill_scratch, and suffers from the same
> problem.
> 
> Fix this by in both cases moving the EHB earlier in the emitted code.
> There's no reason it needs to immediately precede the MFC0 - it simply
> needs to be between the MTC0 & MFC0.
> 
> This bug only affects Cavium Octeon systems which use
> build_fast_tlb_refill_handler().

Applied to mips-fixes.

> commit b42aa3fd5957
> https://git.kernel.org/mips/c/b42aa3fd5957
> 
> Signed-off-by: Paul Burton <paulburton@...nel.org>
> Fixes: 0b24cae4d535 ("MIPS: Add missing EHB in mtc0 -> mfc0 sequence.")

Thanks,
    Paul

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