On Fri, May 22, 2020 at 11:27:38AM +0100, Igor Druzhinin wrote: > On 22/05/2020 11:23, Roger Pau Monné wrote: > > 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? > > It's not "only MMIO" - it's just MMIO area is hit in my particular case. > I'd prefer this fix to address the general issue otherwise for SVM > we would have to write handlers in hvm_hap_nested_page_fault() for > every case as soon as we hit it.
Hm, I'm not sure I agree. p2m memory types are limited, and IMO we want to have strict control about how they are handled. hvm_hap_nested_page_fault is already full of special casing for each memory type for that reason. That being said, I also don't like the fact that logdity is handled differently between EPT and NPT, as on EPT it's handled as a misconfig while on NPT it's handled as a violation. Thanks, Roger.
