RE: [PATCH for-4.14 v4] x86/tlb: fix assisted flush usage

[Paul Durrant <xadimgnik@xxxxxxxxx>]

> -----Original Message-----
> From: Julien Grall <julien@xxxxxxx>
> Sent: 30 June 2020 13:48
> To: Roger Pau Monne <roger.pau@xxxxxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx
> Cc: paul@xxxxxxx; Stefano Stabellini <sstabellini@xxxxxxxxxx>; Volodymyr 
> Babchuk
> <Volodymyr_Babchuk@xxxxxxxx>; Andrew Cooper <andrew.cooper3@xxxxxxxxxx>; 
> George Dunlap
> <george.dunlap@xxxxxxxxxx>; Ian Jackson <ian.jackson@xxxxxxxxxxxxx>; Jan 
> Beulich <jbeulich@xxxxxxxx>;
> Wei Liu <wl@xxxxxxx>
> Subject: Re: [PATCH for-4.14 v4] x86/tlb: fix assisted flush usage
> 
> Hi Roger,
> 
> On 26/06/2020 16:57, Roger Pau Monne wrote:
> > Commit e9aca9470ed86 introduced a regression when avoiding sending
> > IPIs for certain flush operations. Xen page fault handler
> > (spurious_page_fault) relies on blocking interrupts in order to
> > prevent handling TLB flush IPIs and thus preventing other CPUs from
> > removing page tables pages. Switching to assisted flushing avoided such
> > IPIs, and thus can result in pages belonging to the page tables being
> > removed (and possibly re-used) while __page_fault_type is being
> > executed.
> >
> > Force some of the TLB flushes to use IPIs, thus avoiding the assisted
> > TLB flush. Those selected flushes are the page type change (when
> > switching from a page table type to a different one, ie: a page that
> > has been removed as a page table) and page allocation. This sadly has
> > a negative performance impact on the pvshim, as less assisted flushes
> > can be used. Note the flush in grant-table code is also switched to
> > use an IPI even when not strictly needed. This is done so that a
> > common arch_flush_tlb_mask can be introduced and always used in common
> > code.
> >
> > Introduce a new flag (FLUSH_FORCE_IPI) and helper to force a TLB flush
> > using an IPI (flush_tlb_mask_sync, x86 only). Note that the flag is
> > only meaningfully defined when the hypervisor supports PV or shadow
> > paging mode, as otherwise hardware assisted paging domains are in
> > charge of their page tables and won't share page tables with Xen, thus
> > not influencing the result of page walks performed by the spurious
> > fault handler.
> >
> > Just passing this new flag when calling flush_area_mask prevents the
> > usage of the assisted flush without any other side effects.
> >
> > Note the flag is not defined on Arm.
> >
> > Fixes: e9aca9470ed86 ('x86/tlb: use Xen L0 assisted TLB flush when 
> > available')
> > Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> > Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> 
> Acked-by: Julien Grall <jgrall@xxxxxxxxxx>
> 

And...

Release-acked-by: Paul Durrant <paul@xxxxxxx>