From mboxrd@z Thu Jan 1 00:00:00 1970 From: will.deacon@arm.com (Will Deacon) Date: Mon, 5 Feb 2018 16:41:30 +0000 Subject: [PATCH v2 7/9] arm64: entry: Reword comment about post_ttbr_update_workaround In-Reply-To: References: <1517227200-20412-1-git-send-email-will.deacon@arm.com> <1517227200-20412-8-git-send-email-will.deacon@arm.com> Message-ID: <20180205164129.GE10172@arm.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Sat, Feb 03, 2018 at 11:15:59AM +0000, Ard Biesheuvel wrote: > On 29 January 2018 at 11:59, Will Deacon wrote: > > We don't fully understand the Cavium ThunderX erratum, but it appears > > that mapping the kernel as nG can lead to horrible consequences such as > > attempting to execute userspace from kernel context. Since kpti isn't > > enabled for these CPUs anyway, simplify the comment justifying the lack > > of post_ttbr_update_workaround in the exception trampoline. > > > > Signed-off-by: Will Deacon > > --- > > arch/arm64/kernel/entry.S | 12 ++---------- > > 1 file changed, 2 insertions(+), 10 deletions(-) > > > > diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S > > index b34e717d7597..fbe1444324b3 100644 > > --- a/arch/arm64/kernel/entry.S > > +++ b/arch/arm64/kernel/entry.S > > @@ -1013,16 +1013,8 @@ alternative_else_nop_endif > > orr \tmp, \tmp, #USER_ASID_FLAG > > msr ttbr1_el1, \tmp > > /* > > - * We avoid running the post_ttbr_update_workaround here because the > > - * user and kernel ASIDs don't have conflicting mappings, so any > > - * "blessing" as described in: > > - * > > - * http://lkml.kernel.org/r/56BB848A.6060603 at caviumnetworks.com > > - * > > - * will not hurt correctness. Whilst this may partially defeat the > > - * point of using split ASIDs in the first place, it avoids > > - * the hit of invalidating the entire I-cache on every return to > > - * userspace. > > + * We avoid running the post_ttbr_update_workaround here because > > + * it's only needed by Cavium ThunderX, which doesn't require kpti. > > 'requires KPTI to be disabled' sounds more accurate to me Fair enough. I'll leave this for Catalin to merge in. Will