Re: [Qemu-devel] [PATCH] exec.c: Allow memory region start_addr and region_offset to vary in low bits

[Peter Maydell]

On 5 December 2011 13:40, Avi Kivity <address@hidden> wrote:
> On 12/05/2011 01:01 PM, Peter Maydell wrote:
>> @@ -2677,7 +2674,11 @@ void 
>> cpu_register_physical_memory_log(target_phys_addr_t start_addr,
>>      if (phys_offset == IO_MEM_UNASSIGNED) {
>>          region_offset = start_addr;
>>      }
>> -    region_offset &= TARGET_PAGE_MASK;
>> +    /* Adjust the region offset to account for the start_addr possibly
>> +     * not being page aligned, so we end up passing the IO functions
>> +     * the true offset from the start of the region.
>> +     */
>> +    region_offset -= (start_addr & ~TARGET_PAGE_MASK);
>>      size = (size + TARGET_PAGE_SIZE - 1) & TARGET_PAGE_MASK;
>>      end_addr = start_addr + (target_phys_addr_t)size;
>>
>
> region_offset is added to iotlb in tlb_set_page(), smashing the low bits
> with your change.  It's safe in subpage, since that doesn't happen there.

OK, but we only need to avoid trashing the bottom 5 bits, right?
So we could do
    region_offset -= (start_addr & ~TARGET_PAGE_MASK);
    if (size >= TARGET_PAGE_SIZE) {
        region_offset &= ~0x1F; /* can make this a #define IO_MEM_MASK */
    }

which would allow regions to start on 0x20 granularity, or byte granularity
if they're less than a page in size (and so guaranteed to be subpages only).

-- PMM