Re: Event delivery and "domain blocking" on PVHv2

[Martin Lucina <martin@xxxxxxxxxx>]

On 2020-06-19 13:21, Andrew Cooper wrote:
> On 19/06/2020 11:28, Martin Lucina wrote:
> > RIP 0x209997 is the 'hlt' instruction in
> > mirage_xen_evtchn_block_domain() so we are indeed blocking waiting for
> > events to show up.
> I can't find this in the code, but it might be an x86-ism you're 
> falling
> over here.
Solo5 only contains the lowest-level bits, and only then those parts 
that
"fit" the responsibility of that layer. The rest is here (WIP, not 
cleaned
up yet):

https://github.com/mato/mirage-xen/tree/xen-pvh-via-solo5

The event channel code, including the function that blocks the domain,
is in lib/bindings/evtchn_stubs.c.

> Its not safe to use hlt with interrupts enabled, unless it is exactly
> `sti; hlt` where the STI instruction transitions the interrupt flag 
> from
> clear to set (i.e. you had interrupts disabled beforehand).
>
> Otherwise you can take the interrupt intended to wake you on the before
> the hlt is executed.
Hmm, so what is the right thing to do when blocking on PVHv2 in this
scenario? Just use SCHEDOP_block? "cli; sti; hlt"? (Tried the latter,
doesn't help).

Martin