On 25/09/2024 7:35 am, Jan Beulich wrote: > On 24.09.2024 20:36, Andrew Cooper wrote: >> On 23/09/2024 2:03 pm, Jan Beulich wrote: >>> On 23.09.2024 12:14, Alejandro Vallejo wrote: >>>> On Fri Sep 20, 2024 at 3:12 PM BST, Roger Pau Monné wrote: >>>>> On Wed, Sep 18, 2024 at 02:05:54PM +0100, Alejandro Vallejo wrote: >>>>>> Moves sti directly after the cr2 read and immediately after the #PF >>>>>> handler. >>>>> I think you need to add some context about why this is needed, iow: >>>>> avoid corrupting %cr2 if a nested 3PF happens. >>>> I can send a v3 with: >>>> >>>> ``` >>>> Hitting a page fault clobbers %cr2, so if a page fault is handled while >>>> handling a previous page fault then %cr2 will hold the address of the >>>> latter >>>> fault rather than the former. This patch makes the page fault path delay >>>> re-enabling IRQs until %cr2 has been read in order to ensure it stays >>>> consistent. >>> And under what conditions would we experience #PF while already processing >>> an earlier #PF? If an interrupt kicks in, that's not supposed to by raising >>> any #PF itself. Which isn't to say that the change isn't worthwhile to make, >>> but it would be nice if it was explicit whether there are active issues, or >>> whether this is merely to be on the safe side going forward. >> My understanding is that this came from code inspection, not an active >> issue. >> >> The same is true for %dr6 and #DB, and MSR_XFD_ERR and #NM. >> >> I think we can safely agree to veto the use of AMX in the #NM handler, >> and IST exceptions don't re-enable interrupts[1], so #PF is the only >> problem case. >> >> Debug keys happen off the back of plain IRQs, and we can get #PF when >> interrogating guest stacks. > Hmm, yes, this looks like a case that is actively being fixed here. Wants > mentioning, likely wants a respective Fixes: tag, and then also wants > backporting.
I'd also like to see at least a brief mention of #DB/#NM and why they're safe. >> Also, I'm far from certain we're safe to >> spurious #PF's from updating Xen mappings, so I think there are a bunch >> of risky corner cases that we might be exposed to. > Spurious #PF are possible, but __page_fault_type() explicitly excludes > the in_irq() case. Right, but %cr2 is clobbered when the spurious #PF occurs, whatever __page_fault_type() subsequently decides to do. ~Andrew
