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

[Alexander Graf]

On 10.11.14 13:31, Igor Mammedov wrote:
> On Fri,  7 Nov 2014 22:18:45 +0100
> Alexander Graf <address@hidden> 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: address@hidden
>> Signed-off-by: Alexander Graf <address@hidden>
>> ---
>>  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