Re: [Qemu-devel] [PATCH] extend limit of physical sections number

[Paolo Bonzini]

Il 05/11/2013 01:36, Peter Maydell ha scritto:
> On 27 September 2013 17:49, Amos Kong <address@hidden> wrote:
>>  # qemu -drive file=/disk0,if=none,id=v0,format=qcow2 \
>>  -device virtio-blk-pci,drive=v0,id=v00,multifunction=on,addr=0x04.0
>>  ....
>>
>> Launching guest with more than 32 virtio-blk disks,
>> qemu will crash, because there are too many BARs.
>>
>> This patch brings the limit of non-tcg up by a factor
>> of 8 (32767 / 4096), i.e. 32*8 = 256.
>>
>> Signed-off-by: Paolo Bonzini <address@hidden>
>> Signed-off-by: Amos Kong <address@hidden>
>> ---
>>  exec.c | 17 ++++++++++++-----
>>  1 file changed, 12 insertions(+), 5 deletions(-)
>>
>> diff --git a/exec.c b/exec.c
>> index 5aef833..f639c01 100644
>> --- a/exec.c
>> +++ b/exec.c
>> @@ -763,11 +763,18 @@ void phys_mem_set_alloc(void *(*alloc)(ram_addr_t))
>>
>>  static uint16_t phys_section_add(MemoryRegionSection *section)
>>  {
>> -    /* The physical section number is ORed with a page-aligned
>> -     * pointer to produce the iotlb entries.  Thus it should
>> -     * never overflow into the page-aligned value.
>> -     */
>> -    assert(next_map.sections_nb < TARGET_PAGE_SIZE);
>> +    if (tcg_enabled()) {
>> +        /* The physical section number is ORed with a page-aligned
>> +         * pointer to produce the iotlb entries.  Thus it should
>> +         * never overflow into the page-aligned value.
>> +         */
>> +        assert(next_map.sections_nb < TARGET_PAGE_SIZE);
>> +    } else {
>> +        /* For KVM or Xen we can use the full range of the ptr field
>> +         * in PhysPageEntry.
>> +         */
>> +        assert(next_map.sections_nb < SHRT_MAX);
>> +    }
> 
> This looks really weird. Why should the memory subsystem
> care whether we're using TCG or KVM or Xen?

Because only TCG stores the section number in the low bits of the iotlb
entry.  This is exactly what is explained in the comments.

Paolo