Re: [PATCH 4/5] OvmfPkg/XenPlatformPei: Calibrate APIC timer frequency

["Anthony PERARD" <anthony.perard@citrix.com>]

On Thu, Jan 30, 2020 at 10:12:51AM +0100, Roger Pau Monné wrote:
> On Wed, Jan 29, 2020 at 12:12:34PM +0000, Anthony PERARD wrote:
> > +  Parameters.domid = DOMID_SELF;
> > +  Parameters.gpfn = (UINTN) PagePtr >> EFI_PAGE_SHIFT;
> > +  ReturnCode = XenHypercallMemoryOp (XENMEM_remove_from_physmap, &Parameters);
> 
> I'm afraid this will leave a hole in the p2m, and hence freeing the
> page back to OVMF is wrong (I assume this is what FreePages does in
> CalibrateLapicTimer), as the physical address would be unpopulated
> after the call to XENMEM_remove_from_physmap.

I guess there's more refactoring to do in OVMF, there are other's of
this kind of call, mostly in the PV drivers, XenBusDxe.

> > +
> > +  SharedInfo = AllocatePages (1);
> 
> Hm, it's not the best approach to use a regular memory page to map the
> shared info: mapping it is very likely to cause superpage shattering,
> and once unmapped it leaves a hole in the p2m.

:-(

> As a reference you could map the shared info page at maximum physical
> address allowed (after checking it's not populated) like Wei is doing
> for the Xen on HyperV work.

I'll look at what can be done with OVMF.

Is there some kind of information that Xen could give? Or is it all
information that OVMF should keep track of? Or if Xen only have
XENMEM_memory_map, then OVMF already have this information.

Thanks,

-- 
Anthony PERARD