Re: [Qemu-devel] [RFC PATCH 1/3] hw/arm/virt: add hotplug memory support

[Peter Maydell <peter.maydell@linaro.org>]

On 1 August 2016 at 10:14, Igor Mammedov <imammedo@redhat.com> wrote:
> On Mon, 1 Aug 2016 09:13:34 +0100
> Peter Maydell <peter.maydell@linaro.org> wrote:
>> On 1 August 2016 at 08:46, Igor Mammedov <imammedo@redhat.com> wrote:
>> > Base alignment comes from max supported hugepage size,
>>
>> Max hugepage size for any host? (if so, should be defined
>> in a common header somewhere)
>> Max hugepage size for ARM hosts? (if so, why is TCG
>> different from KVM?, and should still be in a common
>> header somewhere)
> It's the same for TCG but it probably doesn't matter much there,
> main usage is to provide better performance with KVM.
>
> So I'd say it's host depended (for x86 it's 1Gb),
> probably other values for ARM and PPC

We probably don't want to make the memory layout depend
on the host architecture, though :-(

>>
>> > while
>> > size alignment should depend on backend's page size
>>
>> Which page size do you have in mind here? TARGET_PAGE_SIZE
>> is often not the right answer, since it doesn't
>> correspond either to the actual page size being used
>> by the host kernel or to the actual page size used
>> by the guest kernel...
> alignment comes from here: memory_region_get_alignment()
>
> exec:c
>    MAX(page_size, QEMU_VMALLOC_ALIGN)
> so it's either backend's page size or a min chunk QEMU
> allocates memory to make KVM/valgrind/whatnot happy.

Since that's always larger than TARGET_PAGE_SIZE
why are we checking for an alignment here that's
not actually sufficient to make things work?

thanks
-- PMM