Re: [PATCH RFCv2 9/9] arm64: Support async page fault

[Marc Zyngier]

On 2020-05-29 00:02, Gavin Shan wrote:
> Hi Paolo,
>
> On 5/28/20 8:48 PM, Paolo Bonzini wrote:
> > On 28/05/20 08:14, Gavin Shan wrote:
> > > > - for x86 we're also thinking of initiating the page fault from the
> > > > exception handler, rather than doing so from the hypervisor before
> > > > injecting the exception. If ARM leads the way here, we would do our
> > > > best to share code when x86 does the same.
> > >
> > > Sorry, Paolo, I don't follow your idea here. Could you please provide
> > > more details?
> >
> > The idea is to inject stage2 page faults into the guest even before 
> > the
> > host starts populates the page.  The guest then invokes a hypercall,
> > telling the host to populate the page table and inject the 'page 
> > ready'
> > event (interrupt) when it's done.
> >
> > For x86 the advantage is that the processor can take care of raising 
> > the
> > stage2 page fault in the guest, so it's faster.
> I think there might be too much overhead if the page can be populated
> quickly by host. For example, it's fast to populate the pages if swapin
> isn't involved.
>
> If I'm correct enough, it seems arm64 doesn't have similar mechanism,
> routing stage2 page fault to guest.

Indeed, this isn't a thing on arm64. Exception caused by a S2 fault are
always routed to EL2.

        M.
--
Jazz is not dead. It just smells funny...