[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: Ping: [PATCH v2] x86/PV: make post-migration page state consistent
On 23.11.2020 13:26, Andrew Cooper wrote: > On 20/11/2020 12:48, Jan Beulich wrote: >> On 04.11.2020 08:56, Jan Beulich wrote: >>> When a page table page gets de-validated, its type reference count drops >>> to zero (and PGT_validated gets cleared), but its type remains intact. >>> XEN_DOMCTL_getpageframeinfo3, therefore, so far reported prior usage for >>> such pages. An intermediate write to such a page via e.g. >>> MMU_NORMAL_PT_UPDATE, however, would transition the page's type to >>> PGT_writable_page, thus altering what XEN_DOMCTL_getpageframeinfo3 would >>> return. In libxc the decision which pages to normalize / localize >>> depends solely on the type returned from the domctl. As a result without >>> further precautions the guest won't be able to tell whether such a page >>> has had its (apparent) PTE entries transitioned to the new MFNs. >>> >>> Add a check of PGT_validated, thus consistently avoiding normalization / >>> localization in the tool stack. >>> >>> Also use XEN_DOMCTL_PFINFO_NOTAB in the variable's initializer instead >>> open coding it. >>> >>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx> >>> --- >>> v2: Don't change type's type. >> Ping? > > Ping what? There is still nothing addressing my concerns from v1. I did reply to your concerns on Sep 11th, and then replied to this reply of mine another time on Sep 28th. Neither of these got any response from you, hence I had to conclude - after two further pings on v1 - that they were satisfactory to you. Now you say they weren't, but without saying in which way, so I still wouldn't know what to change in the description. On the code change itself you did say "... so this is probably a good change", so I was further understanding that your concern is merely with the description. Maybe I misunderstood this aspect, too? > To re-iterate - this is a very subtle change, in a very complicated > piece of migration. As the problems described do not manifest in > practice, it is vital to understand why. Until now it has been my understanding that they just don't happen to manifest, because guests know to behave themselves (read: pin, first and foremost, all their page tables, which means we wouldn't in practice run into ones with an in-flight state change). Jan
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