Re: Increase GICV size to 64KB so !4KB page-size kernels are able to initialize KVM

[Marc Zyngier <marc.zyngier@arm.com>]

On 23/06/16 04:17, Itaru Kitayama wrote:
> On 6/22/16 9:20 PM, Marc Zyngier wrote:
>> I'm only using DT, which works just fine on Overdrive, even with 64kB
>> pages.
>>
>> I think I understand the issue you're having, which is not what I
>> was thinking of. The issue is that because ACPI doesn't tell us
>> anything about the size of the GICV region, we have to assume that
>> it is 8kB, and cannot distinguish a safe platform from an unsafe
>> one, failing the size test on 64kB.
> 
> Is it likely in the future specification of ACPI the size information
> stored in the GIC subtable? In 6.1 or earlier, it seems optional to me.

I don't see any provision for that, and it is already too late (systems
exist in the wild).

> I also had to allow the not page-aligned physical address of GICV, below
> is an excerpt of apic.dsl
> 
> [054h 0084   8]     Virtual GIC Base Address : 00000000E116F000

Right, this is what I was referring to in my initial reply. The problem
is that we cannot know what is actually safe to map into a guest's
address space without any sizing information.

Also, we have the issue of mapping GICV at an offset in a page, which
userspace needs to be made aware of. So even with your hack, it is
unlikely that you'll be able to boot a guest because userspace has only
1/16th probability to map GICV at offset 0xf000.

Doing so requires reporting the page offset to userspace. I had this patch:

http://lists.infradead.org/pipermail/linux-arm-kernel/2014-June/264852.html

but that requires buy-in from QEMU (though I can do the corresponding
kvmtool change).

Thanks,

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