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
