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. 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
