Re: [PATCH v4 10/16] cputlb: Partially inline memory_region_section_get_iotlb

[David Hildenbrand <david@redhat.com>]

On 25.09.19 19:55, Richard Henderson wrote:
> On 9/24/19 12:59 AM, David Hildenbrand wrote:
>>> +    is_ram = memory_region_is_ram(section->mr);
>>> +    is_romd = memory_region_is_romd(section->mr);
>>> +
>>> +    if (is_ram || is_romd) {
>>> +        /* RAM and ROMD both have associated host memory. */
>>>          addend = (uintptr_t)memory_region_get_ram_ptr(section->mr) + xlat;
>>> +    } else {
>>> +        /* I/O does not; force the host address to NULL. */
>>> +        addend = 0;
>>> +    }
>>> +
>>> +    write_address = address;
>>
>> I guess the only "suboptimal" change is that you now have two checks for
>> "prot & PAGE_WRITE" twice in the case of ram instead of one.
> 
> It's a single bit test on a register operand -- as cheap as can be.  If you
> look at the entire code, there *must* be more than one test.  You can rearrange
> the code to choose exactly where those tests are, but you'll have to have them
> somewhere.
> 
>>> +        /* I/O or ROMD */
>>> +        iotlb = memory_region_section_get_iotlb(cpu, section) + xlat;
>>> +        /*
>>> +         * Writes to romd devices must go through MMIO to enable write.
>>> +         * Reads to romd devices go through the ram_ptr found above,
>>> +         * but of course reads to I/O must go through MMIO.
>>> +         */
>>> +        write_address |= TLB_MMIO;
>>
>> ... and here you calculate write_address even if probably unused.
> 
> Well... while the page might not be writable (but I'd bet that it is -- I/O
> memory is almost never read-only), and therefore write_address is technically
> unused, the variable is practically used in the next line:
> 
>     if (!is_romd) {
>         address = write_address
>     }
> 
> which will compile to a conditional move.
> 
>> Can your move the calculation of the write_address completely into the
>> "prot & PAGE_WRITE" case below?
> 
> We'd prefer not to, since the code below is within the cpu tlb lock region.
> We'd prefer to keep all of the expensive operations outside that.

Makes all sense to me then and looks sane :)

> 
> 
> r~
> 


-- 

Thanks,

David / dhildenb