Re: [Xen-devel] [PATCH v2 16/17] libxc/xc_dom_arm: Copy ACPI tables to guest space

[Julien Grall <julien.grall@xxxxxxx>]

Hi,

On 14/07/16 14:37, Stefano Stabellini wrote:
> On Wed, 13 Jul 2016, Julien Grall wrote:
> > Hello,
> >
> > On 12/07/2016 17:58, Boris Ostrovsky wrote:
> > > On 07/12/2016 12:10 PM, Julien Grall wrote:
> > > > On 12/07/2016 16:08, Boris Ostrovsky wrote:
> > > > > On 07/12/2016 10:57 AM, Shannon Zhao wrote:
> > > > It will affect some others part of the guest if we don't increment the
> > > > "maxmem" requested by the user. For ARM the ACPI blob will be exposed
> > > > at a specific address that is outside of the guest RAM (see the guest
> > > > memory layout in public/arch-arm.h).
> > > >
> > > > We chose this solution over putting in the RAM because the ACPI tables
> > > > are not easily relocatable (compare to the device tree, initrd and
> > > > kernel) so we could not take advantage of superpage in both stage-2
> > > > (hypervisor) and stage-1 (kernel) page table.
> > > Maybe this is something ARM-specific then. For x86 we will want to keep
> > > maxmem unchanged.
> > I don't think what I described in my previous mail is ARM-specific. The
> > pressure will be more important on the TLBs, if Xen does not use superpage in
> > the stage 2 page tables (i.e EPT for x86) no matter the architecture.
> >
> > IHMO, this seems to be a bigger drawback compare to add few more kilobytes to
> > maxmem in the toolstack for the ACPI blob. You will loose them when creating
> > the intermediate page table in any case.
> I agree with Julien. On ARM we have to increase maxmem because I don't
> think that shattering a superpage is acceptable for just a few KBs. In
> fact, it's not much about increasing maxmem, but it's about keeping the
> allocation of guest memory to the value passed by the user in "memory",
> so that it can be done in the most efficient way possible. (I am
> assuming users are going to allocate VMs of 2048MB, rather than 2049MB.)
>
> I wouldn't want to end up adding to the performance tuning page on the
> wiki "Make sure to add 1 more MB to the memory of your VM to get the
> most out of the system."
>
> I know that the location of the ACPI blob on x86 is different in guest
> memory space, but it seems to me that the problem would be the same. Do
> you have 1 gigabyte pages in stage-2 on x86? If so, I would think twice
> about this. Otherwise, if you only have 4K and 2MB allocations, then it
> might not make that much of a difference.
Looking at the x86 code, 1 gigabyte pages seems to be supported.

Boris, do you have any opinions on this?

Regards,

--
Julien Grall

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