Re: [Qemu-devel] VFIO use of HOST_PAGE_ALIGN

[Peter Crosthwaite]

On Wed, Jun 3, 2015 at 4:16 AM, Alexey Kardashevskiy <address@hidden> wrote:
> On 06/01/2015 04:27 AM, Peter Crosthwaite wrote:
>>
>> On Sun, May 31, 2015 at 6:34 AM, Alexey Kardashevskiy <address@hidden>
>> wrote:
>>>
>>> On 05/27/2015 01:22 AM, Alex Williamson wrote:
>>>>
>>>>
>>>> [cc +alexey]
>>>>
>>>> On Mon, 2015-05-25 at 00:48 -0700, Peter Crosthwaite wrote:
>>>>>
>>>>>
>>>>> Hi Alex and all,
>>>>>
>>>>> I am working on a patch series to enable multiple CPU architectures to
>>>>> run at once. It's a long story, but I have hit a snag in hw/vfio/pci.c
>>>>> which AFAICS is the only in-tree system-mode use of HOST_PAGE_ALIGN
>>>>> (all usermode code is a non-issue as not looking to support user-mode
>>>>> multi-arch yet).
>>>>>
>>>>> The problem I face, is that this macro depends on translate-all.c's
>>>>> qemu_host_page_size which in turn, depends on TARGET_PAGE_SIZE. I'm
>>>>> hoping that one day, TARGET_PAGE_SIZE will be a variable and the users
>>>>> of it will know to get the correct value depending on their CPU
>>>>> specific code location. vfio is the only one I can't handle. My
>>>>> knowledge on vfio is near-0, but my thinking is, since this is not
>>>>> arch specific code can we instead use the raw host page alignment
>>>>> rather that the CPU arch specific one?
>>>
>>>
>>>
>>> What is "raw host page" here? I thought qemu_host_page_size is the one,
>>> where does it depend on TARGET_PAGE_SIZE?
>>>
>>>
>>
>> In translate-all.c:
>>
>> void page_size_init(void)
>> {
>>      /* NOTE: we can always suppose that qemu_host_page_size >=
>>         TARGET_PAGE_SIZE */
>>      qemu_real_host_page_size = getpagesize();
>>      if (qemu_host_page_size == 0) {
>>          qemu_host_page_size = qemu_real_host_page_size;
>>      }
>>      if (qemu_host_page_size < TARGET_PAGE_SIZE) {
>>          qemu_host_page_size = TARGET_PAGE_SIZE;
>>      }
>>      qemu_host_page_mask = ~(qemu_host_page_size - 1);
>> }
>>
>> It is clamped to be at least as big as a the TARGET_PAGE_SIZE.
>
>
>
> Ah, just this one. TARGET_PAGE_SIZE is 4K for PPC64 and the actual page size
> can be 4K and 64K so that branch never works for PPC64.
>
>
>>>> I think we could replace our use of HOST_PAGE_ALIGN with something based
>>>> only on the host's getpagesize().  I don't see that we really care about
>>>> the target page size for this usage.  Alexey, I think you're the only
>>>> arch where host and target page sizes can actually be different, do you
>>>> agree?  Thanks,
>>>
>>>
>>>
>>> Strongly agree. Where it really matters (MSIX), it is already
>>> qemu_host_page_size and HOST_PAGE_ALIGN and I am a bit scared by that
>>> "raw
>>> host page alignment" :)
>>>
>>
>> So the change would be an alternate macro based on
>> qemu_real_host_page_size which is what I am calling "raw host page
>> alignment".
>
>
> Good, I like it. Cannot it be just a qemu_host_page_size? It is a bit
> confusing to have both qemu_host_page_size and qemu_real_host_page_size when
> even the first name suggests it is something non-static and sort of real :)
>

Yes based on this, HOST_PAGE_ALIGN is badly named. It is really
aligning to both host and target. Some rename options:

QEMU_PAGE_ALIGN
HOST_TARGET_PAGE_ALIGN
QEMU_HOST_PAGE_ALIGN
or just unqualified PAGE_ALIGN?

Regards,
Peter

>
>
> --
> Alexey
>