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

["Roger Pau Monné" <roger.pau@citrix.com>]

On Mon, Jun 22, 2020 at 03:51:24PM +0200, Jan Beulich wrote:
> On 22.06.2020 15:24, Roger Pau Monné wrote:
> > On Mon, Jun 22, 2020 at 01:07:10PM +0200, Jan Beulich wrote:
> >> On 22.06.2020 11:31, Roger Pau Monné wrote:
> >>> On Fri, Jun 19, 2020 at 04:06:55PM +0200, Jan Beulich wrote:
> >>>> On 18.06.2020 18:04, 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.
> >>>>>
> >>>>> Introduce a new flag (FLUSH_FORCE_IPI) and helper to force a TLB flush
> >>>>> using an IPI (flush_tlb_mask_sync). Note that the flag is only
> >>>>> meaningfully defined when the hypervisor supports PV mode, as
> >>>>> otherwise translated 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.
> >>>>
> >>>> Is this true for shadow mode? If a page shadowing a guest one was
> >>>> given back quickly enough to the allocator and then re-used, I think
> >>>> the same situation could in principle arise.
> >>>
> >>> Hm, I think it's not applicable to HVM shadow mode at least, because
> >>> CR3 is switched as part of vmentry/vmexit, and the page tables are not
> >>> shared between Xen and the guest, so there's no way for a HVM shadow
> >>> guest to modify the page-tables while Xen is walking them in
> >>> spurious_page_fault (note spurious_page_fault is only called when the
> >>> fault happens in non-guest context).
> >>
> >> I'm afraid I disagree, because of shadow's use of "linear page tables".
> > 
> > You will have to bear with me, but I don't follow.
> > 
> > Could you provide some pointers at how/where the shadow (I assume
> > guest controlled) "linear page tables" are used while in Xen
> > context?
> 
> See config.h:
> 
> /* Slot 258: linear page table (guest table). */
> #define LINEAR_PT_VIRT_START    (PML4_ADDR(258))
> #define LINEAR_PT_VIRT_END      (LINEAR_PT_VIRT_START + PML4_ENTRY_BYTES)
> /* Slot 259: linear page table (shadow table). */
> #define SH_LINEAR_PT_VIRT_START (PML4_ADDR(259))
> #define SH_LINEAR_PT_VIRT_END   (SH_LINEAR_PT_VIRT_START + PML4_ENTRY_BYTES)
> 
> These linear page tables exist in the Xen page tables at basically
> all times as long as a shadow guest's vCPU is in context. They're
> there to limit the overhead of accessing guest page tables and
> their shadows from inside Xen.

Oh, I have to admit I know very little about all this, and I'm not
able to find a description of how this is to be used.

I think the shadow linear page tables should be per-pCPU, and hence
they cannot be modified by the guest while a spurious page fault is
being processed? (since the vCPU running on the pCPU is in Xen
context).

Thanks, Roger.