[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH v2] x86/traps: Re-enable interrupts after reading cr2 in the #PF handler
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. Furthermore, the patch preserves the invariant of "IRQs are only re-enabled if they were enabled in the interrupted context" in order to not break IRQs-off faulting contexts. ``` > > > While in the area, remove redundant q suffix to a movq in entry.S > > > > Signed-off-by: Alejandro Vallejo <alejandro.vallejo@xxxxxxxxx> > > Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx> Thanks > > One nit below. > > > --- > > Got lost alongside other patches. Here's the promised v2. > > > > pipeline: > > https://gitlab.com/xen-project/people/agvallejo/xen/-/pipelines/1458699639 > > v1: > > https://lore.kernel.org/xen-devel/20240911145823.12066-1-alejandro.vallejo@xxxxxxxxx/ > > > > v2: > > * (cosmetic), add whitespace after comma > > * Added ASSERT(local_irq_is_enabled()) to do_page_fault() > > * Only re-enable interrupts if they were enabled in the interrupted > > context. > > --- > > xen/arch/x86/traps.c | 8 ++++++++ > > xen/arch/x86/x86_64/entry.S | 20 ++++++++++++++++---- > > 2 files changed, 24 insertions(+), 4 deletions(-) > > > > diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c > > index 708136f62558..a9c2c607eb08 100644 > > --- a/xen/arch/x86/traps.c > > +++ b/xen/arch/x86/traps.c > > @@ -1600,6 +1600,14 @@ void asmlinkage do_page_fault(struct cpu_user_regs > > *regs) > > > > addr = read_cr2(); > > > > + /* > > + * Don't re-enable interrupts if we were running an IRQ-off region when > > + * we hit the page fault, or we'll break that code. > > + */ > > + ASSERT(!local_irq_is_enabled()); > > + if ( regs->flags & X86_EFLAGS_IF ) > > + local_irq_enable(); > > + > > /* fixup_page_fault() might change regs->error_code, so cache it here. > > */ > > error_code = regs->error_code; > > > > diff --git a/xen/arch/x86/x86_64/entry.S b/xen/arch/x86/x86_64/entry.S > > index b8482de8ee5b..218e5ea85efb 100644 > > --- a/xen/arch/x86/x86_64/entry.S > > +++ b/xen/arch/x86/x86_64/entry.S > > @@ -844,9 +844,9 @@ handle_exception_saved: > > #elif !defined(CONFIG_PV) > > ASSERT_CONTEXT_IS_XEN > > #endif /* CONFIG_PV */ > > - sti > > -1: movq %rsp,%rdi > > - movzbl UREGS_entry_vector(%rsp),%eax > > +.Ldispatch_handlers: > > Maybe 'dispatch_exception', since it's only exceptions that are > handled here? dispatch_handlers seems a bit too generic, but no strong > opinion. Sure, anything would be better than "1:" > > Thanks, Roger. Cheers, Alejandro
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