On Mon, 10 Nov 2014 14:16:58 +0100 Alexander Graf <agraf@xxxxxxx> wrote: > > > On 10.11.14 13:31, Igor Mammedov wrote: > > On Fri, 7 Nov 2014 22:18:45 +0100 > > Alexander Graf <agraf@xxxxxxx> wrote: > > > >> Memory slots have to be page aligned to get entered into KVM. There > >> is existing logic that tries to ensure that we pad memory slots that > >> are not page aligned to the biggest region that would still fit in the > >> alignment requirements. > >> > >> Unfortunately, that logic is broken. It tries to calculate the start > >> offset based on the region size. > >> > >> Fix up the logic to do the thing it was intended to do and document it > >> properly in the comment above it. > >> > >> With this patch applied, I can successfully run an e500 guest with more > >> than 3GB RAM (at which point RAM starts overlapping subpage memory regions). > >> > >> Cc: qemu-stable@xxxxxxxxxx > >> Signed-off-by: Alexander Graf <agraf@xxxxxxx> > >> --- > >> kvm-all.c | 6 ++++-- > >> 1 file changed, 4 insertions(+), 2 deletions(-) > >> > >> diff --git a/kvm-all.c b/kvm-all.c > >> index 44a5e72..596e7ce 100644 > >> --- a/kvm-all.c > >> +++ b/kvm-all.c > >> @@ -634,8 +634,10 @@ static void kvm_set_phys_mem(MemoryRegionSection *section, bool add) > >> unsigned delta; > >> > >> /* kvm works in page size chunks, but the function may be called > >> - with sub-page size and unaligned start address. */ > >> - delta = TARGET_PAGE_ALIGN(size) - size; > >> + with sub-page size and unaligned start address. Pad the start > >> + address to next and truncate size to previous page boundary. */ > > I'm a bit confused how it works at all. > > Lets assume that there is no mapped pages that include start_addr, > > then if start_addr were padded to next page, kvm would map it from there > > but the rest of QEMU would still use unaligned start_addr for MemoryRegion > > that isn't even mapped. > > Sorry, I don't understand this paragraph. Memory slots in general are > accelerations for memory access - for MMIO (RAM is usually aligned), KVM > can always exit to QEMU and just do a manual MMIO exit. > > > It would seem that instead of padding up to the next page, start_addr > > should be moved to the start of the page that includes it to make page > > with original start_addr available to guest. > > No, because in that case you would map something as RAM that really > isn't RAM. > > Imagine you have the following memory layout: > > 0x1000 page size > > 1) 0x00000 - 0x10000 RAM > 2) 0x10000 - 0x10100 MMIO > 3) 0x10100 - 0x20000 RAM > > Then you want to map 1) as memory slot and 4) from 0x11000 onwards as > memory slot. so every access to RAM 0x10100-0x11000 which is not represented as memory slot would cause VMEXIT? > > You can't map the page from 0x10000 - 0x11000 as memory slot, because > part of it is MMIO. > > > Alex -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
