On 8/23/18 5:44 PM, Andi Kleen wrote: > On Thu, Aug 23, 2018 at 03:44:18PM +0200, Vlastimil Babka wrote: >> Two users have reported [1] that they have an "extremely unlikely" system >> with more than MAX_PA/2 memory and L1TF mitigation is not effective. In fact >> it's a CPU with 36bits phys limit (64GB) and 32GB memory, but due to holes >> in the e820 map, the main region is almost 500MB over the 32GB limit: > > Ah I see it's a client part with very large DIMMs and someone being > very brave and using that much memory without ECC. Are you trying to mock the users and diminsh their report because of that? >> >> [ 0.000000] BIOS-e820: [mem 0x0000000100000000-0x000000081effffff] usable >> >> Suggestions to use 'mem=32G' to prefer L1TF mitigation while losing the 500MB >> revealed, that there's an off-by-one error in the check in >> l1tf_select_mitigation(). l1tf_pfn_limit() returns the last usable pfn >> (inclusive), but it's more common and hopefully less error-prone to return the >> first pfn that's over limit, so this patch changes that and updates the other >> callers. > > I can see the off by one, but does it really cause the user's problem? > > They will be still over the limit in any case, with or without off-by-one. > > So the description has nothing to do with the fix. Or do I miss something? The off-by-one happens when 'mem=32G' is used to limit the amount memory. It's easier to do "mem=32G" with fixed code than "mem=33554428k" to workaround the error. > > -Andi >
