On Wed, Feb 01, 2017 at 05:59:48PM +0000, Will Deacon wrote: > On Wed, Feb 01, 2017 at 05:49:34PM +0000, Catalin Marinas wrote: > > On Wed, Feb 01, 2017 at 05:41:05PM +0000, Will Deacon wrote: > > > On Wed, Feb 01, 2017 at 05:36:09PM +0000, Catalin Marinas wrote: > > > > On Wed, Feb 01, 2017 at 04:33:58PM +0000, Will Deacon wrote: > > > > > On Wed, Feb 01, 2017 at 11:29:22AM -0500, Christopher Covington wrote: > > > > > > On 01/31/2017 12:56 PM, Marc Zyngier wrote: > > > > > > > Given that all ARMv8 CPUs can support SW_PAN, it is more likely to be > > > > > > > enabled than the ARMv8.1 PAN. I'd vote for supporting the workaround in > > > > > > > that case too, and hope that people do enable the HW version. > > > > > > > > > > > > Okay, I'll do my best to add support for the SW PAN case. I rebased and > > > > > > submitted v6 of the E1009 patch [1] so that it no longer depends on this > > > > > > patch landing first, if you all are inclined to pick it up while work on > > > > > > this E1003 patch continues. > > > > > > > > > > The alternative is not enabling SW_PAN (at runtime) if this errata is > > > > > present, along with a warning stating that hardware-PAN should be > > > > > enabled in kconfig instead. Not sure what distributions will make of that > > > > > though. > > > > > > > > The problem with this patch is that when ARM64_SW_TTBR0_PAN is enabled > > > > and in the absence of hardware PAN (or ARM64_PAN disabled), > > > > cpu_do_switch_mm is no longer called for user process switching, so the > > > > workaround is pretty much useless. > > > > > > Oh, I see what you mean now. > > > > > > > I'm ok with adding the Kconfig dependency below to > > > > QCOM_FALKOR_ERRATUM_1003: > > > > > > > > depends on !ARM64_SW_TTBR0_PAN || ARM64_PAN > > > > > > > > together with a run-time warning if ARM64_SW_TTBR0_PAN is being used. > > > > > > That makes it look like hardware-PAN is the cause of the erratum. > > > > With the right Kconfig comment we could make this clearer. > > It's not just a comment though, the kconfig option for the workaround > will disappear from menuconfig as long as the dependencies aren't met. > The dependency is really that SW_PAN depends on !ERRATUM_1003, but that > doesn't work for the distributions. I agree. > > > Maybe > > > just select ARM64_PAN if the erratum workaround is selected, then > > > runtime warning if we find that the h/w doesn't have PAN but does have > > > the erratum (which should never fire)? > > > > You still need this workaround even if you don't want any PAN (both sw > > and hw PAN disabled). I wouldn't want to select ARM64_PAN since it's not > > a dependency. It's more like if you do need a PAN, make sure you only > > use the hw one. > > True, in the case that all PAN options are disabled we still want this > to work. How about: > > select ARM64_PAN if ARM64_SW_TTBR0_PAN As I replied to myself, the above would work for me as well, so let's go for this. > In fact, what's the reason for supporting SW_PAN and ARM64_PAN as a > config combination? Why not just have "PAN" that enables them both and > uses the hardware feature if it's there? Because SW PAN has a non-trivial performance hit. You would enable SW PAN only if you are paranoid about security. HW PAN, OTOH, is very cheap and I wouldn't want to miss enabling it in a single Image supporting ARMv8.0 and ARMv8.1 just because SW PAN is slow on ARMv8.0. IOW, ARM64_PAN is default y while ARM64_SW_TTBR0_PAN is default n. -- Catalin -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
