On Fri, May 22, 2020 at 11:14:24AM +0100, Igor Druzhinin wrote: > On 22/05/2020 11:08, Roger Pau Monné wrote: > > On Thu, May 21, 2020 at 10:43:58PM +0100, Igor Druzhinin wrote: > >> If a recalculation NPT fault hasn't been handled explicitly in > >> hvm_hap_nested_page_fault() then it's potentially safe to retry - > >> US bit has been re-instated in PTE and any real fault would be correctly > >> re-raised next time. > >> > >> This covers a specific case of migration with vGPU assigned on AMD: > >> global log-dirty is enabled and causes immediate recalculation NPT > >> fault in MMIO area upon access. This type of fault isn't described > >> explicitly in hvm_hap_nested_page_fault (this isn't called on > >> EPT misconfig exit on Intel) which results in domain crash. > > > > Couldn't direct MMIO regions be handled like other types of memory for > > the purposes of logdiry mode? > > > > I assume there's already a path here used for other memory types when > > logdirty is turned on, and hence would seem better to just make direct > > MMIO regions also use that path? > > The proble of handling only MMIO case is that the issue still stays. > It will be hit with some other memory type since it's not MMIO specific. > The issue is that if global recalculation is called, the next hit to > this type will cause a transient fault which will not be handled > correctly after a due fixup by neither of our handlers.
I admit I should go look at the code, but for example RAM p2m types don't require this fix, so I assume there's some different path taken in that case that avoids all this? Ie: when global logdirty is enabled you will start to get nested page faults for every access, yet only direct MMIO types require this fix? Thanks, Roger
