Re: [PATCH v3 3/5] hw/arm/virt: Honor highmem setting when computing the memory map

[Marc Zyngier]

On Fri, 07 Jan 2022 18:48:16 +0000,
Peter Maydell <peter.maydell@linaro.org> wrote:
> 
> On Fri, 7 Jan 2022 at 18:18, Marc Zyngier <maz@kernel.org> wrote:
> > This is a chicken and egg problem: you need the IPA size to compute
> > the memory map, and you need the memory map to compute the IPA
> > size. Fun, isn't it?
> >
> > At the moment, virt_set_memmap() doesn't know about the IPA space,
> > generates a highest_gpa that may not work, and we end-up failing
> > because the resulting VM type is out of bound.
> >
> > My solution to that is to feed the *maximum* IPA size to
> > virt_set_memmap(), compute the memory map there, and then use
> > highest_gpa to compute the actual IPA size that is used to create the
> > VM. By knowing the IPA limit in virt_set_memmap(), I'm able to keep it
> > in check and avoid generating an unusable memory map.
> 
> Is there any reason not to just always create the VM with the
> maximum supported IPA size, rather than trying to create it
> with the smallest IPA size that will work? (ie skip the last
> step of computing the IPA size to create the VM with)

That gives KVM the opportunity to reduce the depth of the S2 page
tables. On HW that supports a large PA space, there is a real
advantage in keeping these shallow if at all possible.

Thanks,

        M.

-- 
Without deviation from the norm, progress is not possible.