On 14.05.2025 13:30, Roger Pau Monné wrote: > On Tue, May 13, 2025 at 03:54:56PM +0200, Jan Beulich wrote: >> On 13.05.2025 15:41, Roger Pau Monné wrote: >>> It's my understanding that the same is possible on native, as the CPU >>> might speculatively pull lines into the cache. So there's no reason >>> for an OS to use wbinvd if wbnoinvd is available? >> >> Speculatively pulling data into the cache is possible only when page >> table entries permit caching. Hence after changing all mappings of a >> certain page to UC, an OS may have a need to ensure that no data of >> this page is left in any cache (and it can't be pulled back in >> speculatively then). > > Is this realistic taking into account the OS is running virtualized? > > At least with Xen there's the direct map, so once context is switched > back to Xen (for example to execute the wbinvd itself) there's no > guarantee the CPU won't speculatively populate the cache with entries > from the direct map.
Well, we've been knowing for a long time that we're not doing things fully correctly there. Once a guest has changed all mappings of a page it owns, we ought to make the direct map one follow suit (or simply unmap it from there). Jan
