Re: [Xen-devel] [PATCH RFC v2] Add SUPPORT.md

[Julien Grall <julien.grall@xxxxxxxxxx>]

Hi,

On 23/10/2017 18:55, Andrew Cooper wrote:
> On 23/10/17 17:22, George Dunlap wrote:
> > On 09/11/2017 06:53 PM, Andrew Cooper wrote:
> > > On 11/09/17 18:01, George Dunlap wrote:
> > > > +    Limit, x86 HVM: 128
> > > > +    Limit, ARM32: 8
> > > > +    Limit, ARM64: 128
> > > > +
> > > > +[XXX Andrew Cooper: Do want to add "Limit-Security" here for some of these?]
> > > 32 for each.  64 vcpu HVM guests can excerpt enough p2m lock pressure to
> > > trigger a 5 second host watchdog timeout.
> > Is that "32 for x86 PV and x86 HVM", or "32 for x86 HVM and ARM64"?  Or
> > something else?
> The former.  I'm not qualified to comment on any of the ARM limits.
That's a good question. On Arm32 the number of vCPUs is limited by the 
GICv2 implementation.
On Arm64, GICv2 platform can only support up to 8 vCPUs. GICv3 is 
theoretically 4096. But it is capped to 128 vCPUs, IIRC it was just to 
match x86.
> There are several non-trivial for_each_vcpu() loops in the domain_kill
> path which aren't handled by continuations.  ISTR 128 vcpus is enough to
> trip a watchdog timeout when freeing pagetables.
On Arm, we have similar for_each_vcpu() in the vGIC code to inject SPIs 
(see vgic_to_sgi). I haven't tried it so far with a high number of 
vCPUs. So I am not sure if we should stick to 128 too. Stefano do you 
have any opinions?
Cheers,

--
Julien Grall

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