Re: [Qemu-devel] [PATCH] kvm: Fix memory slot page alignment logic

[Igor Mammedov <imammedo@redhat.com>]

On Mon, 10 Nov 2014 14:16:58 +0100
Alexander Graf <agraf@suse.de> wrote:

> 
> 
> On 10.11.14 13:31, Igor Mammedov wrote:
> > On Fri,  7 Nov 2014 22:18:45 +0100
> > Alexander Graf <agraf@suse.de> 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@nongnu.org
> >> Signed-off-by: Alexander Graf <agraf@suse.de>
> >> ---
> >>  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