Hi Mark, On 01/30/2017 05:56 AM, Mark Rutland wrote: > Hi, > > On Fri, Jan 27, 2017 at 04:52:23PM -0500, Christopher Covington wrote: >> On 01/27/2017 09:38 AM, Mark Rutland wrote: >>> On Wed, Jan 25, 2017 at 10:52:30AM -0500, Christopher Covington wrote: > >>>> Replacing the above sequence with the one below will ensure that no TLB >>>> entries with an incorrect ASID are used by software. >>>> >>>> write reserved value to TTBRx_EL1[ASID] >>>> ISB >>>> write new value to TTBRx_EL1[BADDR] >>>> ISB >>>> write new value to TTBRx_EL1[ASID] >>>> ISB >>>> >>>> When the above sequence is used, page table entries using the new BADDR >>>> value may still be incorrectly allocated into the TLB using the reserved >>>> ASID. Yet this will not reduce functionality, since TLB entries incorrectly >>>> tagged with the reserved ASID will never be hit by a later instruction. >>> >>> I agree that there should be no explicit accesses to the VAs for these >>> entries. So tasks should not see erroneous VAs, and we shouldn't see >>> synchronous TLB conflict aborts. >>> >>> Regardless, can this allow conflicting TLB entries to be allocated to >>> the reserved ASID? e.g. if one task has a 4K mapping at a given VA, and >>> another has a 2M mapping which covers that VA, can both be allocated >>> into the TLBs under the reserved ASID? >>> >>> Can that have any effect on asynchronous TLB lookups or page table >>> walks, e.g. for speculated accesses? >> >> A speculative access that inserts an entry into the TLB could >> possibly find the conflict but will not signal it. Does that answer >> your question? > > Yes! > > The other case I was worried about was intermediate caching. I take it > the values in TLBs are not used as part of subsequent page table walks? > > If so, the above sounds fine to me. > > Otherwise, we'll need additional TLB maintenance. Errant TLB entries will not be used for any legitimate subsequent page table walks. I have some minor changes which I'll send as v5 based on kernel/git/arm64/linux.git for-next/core. Thanks, Cov -- Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc. Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.
