Re: [PATCH v2] x86/traps: Re-enable interrupts after reading cr2 in the #PF handler

["Alejandro Vallejo" <alejandro.vallejo@cloud.com>]

Hi,

On Tue Sep 24, 2024 at 7:36 PM BST, 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.

That's right. I merely eyeballed it while going through the interrupt dispatch
sequence. This is not a bugfix as much as simply being cautious.

>
> 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,

Agree.

> 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

Could you elaborate here on debug keys? Not sure I understand what you mean.

> interrogating guest stacks.  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.
>
> And I really need to find some time to get FRED working...
>
> ~Andrew
>
> [1] We do re-enable interrupts in order to IPI cpu0 for "clean"
> shutdown, and this explodes in our faces if kexec isn't active and we
> crashed in the middle of context switch.  We really need to not need
> irqs-on in order to shut down.

Why do we need them currently in that path? Regardless, shutdowns would be the
response to aborts (#MC or #DF) rather than #DB, right?

Cheers,
Alejandro