Hi drew, On 2020/12/15 19:20, Andrew Jones wrote: > On Tue, Dec 15, 2020 at 03:19:47PM +0800, Keqian Zhu wrote: >> When handle dirty log, we face qemu_real_host_page_size and >> TARGET_PAGE_SIZE. The first one is the granule of KVM dirty >> bitmap, and the second one is the granule of QEMU dirty bitmap. >> >> Generally speaking, qemu_real_host_page_size >= TARGET_PAGE_SIZE, > > Not just generally speaking, but must be. We have > > /* > * On systems where the kernel can support different base page > * sizes, host page size may be different from TARGET_PAGE_SIZE, > * even with KVM. TARGET_PAGE_SIZE is assumed to be the minimum > * page size for the system though. > */ > assert(TARGET_PAGE_SIZE <= qemu_real_host_page_size); Yes, I noticed it, but my statement (Generally speaking) is not suitable. Thanks for pointing it out.
> > at the top of kvm_init() to enforce it. > > The comment also says TARGET_PAGE_SIZE is assumed to be the minimum, > which is more of a requirement than assumption. And, that requirement > implies that we require all memory regions and base addresses to be > aligned to the maximum possible target page size (in case the target > actually uses something larger). > > Please remove 'Generally speaking' from the commit message. It > implies this change is based on an assumption rather than a rule. > > (It'd be nice if we had these guest memory and TARGET_PAGE_SIZE > requirements better documented and in one place.) Maybe someone could do this :-) > >> so misuse TARGET_PAGE_SIZE to init kvmslot dirty_bmap may waste >> memory. For example, when qemu_real_host_page_size is 64K and >> TARGET_PAGE_SIZE is 4K, this bugfix can save 93.75% memory. >> >> Signed-off-by: Keqian Zhu <zhukeqi...@huawei.com> >> --- >> accel/kvm/kvm-all.c | 6 +++++- >> 1 file changed, 5 insertions(+), 1 deletion(-) >> >> diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c >> index baaa54249d..c5e06288eb 100644 >> --- a/accel/kvm/kvm-all.c >> +++ b/accel/kvm/kvm-all.c >> @@ -620,8 +620,12 @@ static void kvm_memslot_init_dirty_bitmap(KVMSlot *mem) >> * too, in most cases). >> * So for now, let's align to 64 instead of HOST_LONG_BITS here, in >> * a hope that sizeof(long) won't become >8 any time soon. >> + * >> + * Note: the granule of kvm dirty log is qemu_real_host_page_size. >> + * And mem->memory_size is aligned to it (otherwise this mem can't >> + * be registered to KVM). >> */ >> - hwaddr bitmap_size = ALIGN(((mem->memory_size) >> TARGET_PAGE_BITS), >> + hwaddr bitmap_size = ALIGN(mem->memory_size / qemu_real_host_page_size, >> /*HOST_LONG_BITS*/ 64) / 8; >> mem->dirty_bmap = g_malloc0(bitmap_size); >> } >> -- >> 2.23.0 >> >> > > Besides the commit message > > Reviewed-by: Andrew Jones <drjo...@redhat.com> > OK, I will correct it and send v2 soon. Cheers, Keqian > > Thanks, > drew > > . >
