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

[Igor Mammedov <imammedo@redhat.com>]

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.

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.

> +    delta = (TARGET_PAGE_SIZE - (start_addr & ~TARGET_PAGE_MASK));
> +    delta &= ~TARGET_PAGE_MASK;
>      if (delta > size) {
>          return;
>      }