Re: [PATCH v1 1/5] KVM: arm64: Enable ring-based dirty memory tracking

[Peter Xu <peterx@redhat.com>]

Hi, Gavin,

On Mon, Aug 22, 2022 at 11:58:20AM +1000, Gavin Shan wrote:
> > For context, the documentation says:
> > 
> > <quote>
> > - if KVM_CAP_DIRTY_LOG_RING is available, a number of pages at
> >    KVM_DIRTY_LOG_PAGE_OFFSET * PAGE_SIZE. [...]
> > </quote>
> > 
> > What is the reason for picking this particular value?
> > 
> 
> It's inherited from x86. I don't think it has to be this particular value.
> The value is used to distinguish the region's owners like kvm_run, KVM_PIO_PAGE_OFFSET,
> KVM_COALESCED_MMIO_PAGE_OFFSET, and KVM_DIRTY_LOG_PAGE_OFFSET.
> 
> How about to have 2 for KVM_DIRTY_LOG_PAGE_OFFSET in next revision?
> The virtual area is cheap, I guess it's also nice to use x86's
> pattern to have 64 for KVM_DIRTY_LOG_PAGE_OFFSET.
> 
>     #define KVM_COALESCED_MMIO_PAGE_OFFSET   1
>     #define KVM_DIRTY_LOG_PAGE_OFFSET        2

It was chosen not to be continuous of previous used offset because it'll be
the 1st vcpu region that can cover multiple & dynamic number of pages.  I
wanted to leave the 3-63 (x86 used offset 2 already) for small fields so
they can be continuous, which looks a little bit cleaner.

Currently how many pages it'll use depends on the size set by the user,
though there's a max size limited by KVM_DIRTY_RING_MAX_ENTRIES, which is a
maximum of 1MB memory.

So I think setting it to 2 is okay, as long as we keep the rest 1MB address
space for the per-vcpu ring structure, so any new vcpu fields (even if only
1 page will be needed) need to be after that maximum size of the ring.

Thanks,

-- 
Peter Xu