Re: [PATCH 2/6] qemu-kvm: Modify and introduce wrapper functions to access phys_ram_dirty.

[Anthony Liguori <anthony@codemonkey.ws>]

On 03/16/2010 08:57 AM, Avi Kivity wrote:
> On 03/16/2010 03:51 PM, Anthony Liguori wrote:
>> On 03/16/2010 08:29 AM, Avi Kivity wrote:
>>> On 03/16/2010 03:17 PM, Yoshiaki Tamura wrote:
>>>> Avi Kivity wrote:
>>>>> On 03/16/2010 12:53 PM, Yoshiaki Tamura wrote:
>>>>>> Modifies wrapper functions for byte-based phys_ram_dirty bitmap to
>>>>>> bit-based phys_ram_dirty bitmap, and adds more wrapper functions to
>>>>>> prevent
>>>>>> direct access to the phys_ram_dirty bitmap.
>>>>>
>>>>>> +
>>>>>> +static inline int cpu_physical_memory_get_dirty_flags(ram_addr_t 
>>>>>> addr)
>>>>>> +{
>>>>>> + unsigned long mask;
>>>>>> + int index = (addr>> TARGET_PAGE_BITS) / HOST_LONG_BITS;
>>>>>> + int offset = (addr>> TARGET_PAGE_BITS)& (HOST_LONG_BITS - 1);
>>>>>> + int ret = 0;
>>>>>> +
>>>>>> + mask = 1UL<< offset;
>>>>>> + if (phys_ram_vga_dirty[index]& mask)
>>>>>> + ret |= VGA_DIRTY_FLAG;
>>>>>> + if (phys_ram_code_dirty[index]& mask)
>>>>>> + ret |= CODE_DIRTY_FLAG;
>>>>>> + if (phys_ram_migration_dirty[index]& mask)
>>>>>> + ret |= MIGRATION_DIRTY_FLAG;
>>>>>> +
>>>>>> + return ret;
>>>>>> }
>>>>>>
>>>>>> static inline int cpu_physical_memory_get_dirty(ram_addr_t addr,
>>>>>> int dirty_flags)
>>>>>> {
>>>>>> - return phys_ram_dirty[addr>> TARGET_PAGE_BITS]& dirty_flags;
>>>>>> + return cpu_physical_memory_get_dirty_flags(addr)& dirty_flags;
>>>>>> }
>>>>>
>>>>> This turns one cacheline access into three. If the dirty bitmaps 
>>>>> were in
>>>>> an array, you could do
>>>>>
>>>>> return dirty_bitmaps[dirty_index][addr >> (TARGET_PAGE_BITS +
>>>>> BITS_IN_LONG)] & mask;
>>>>>
>>>>> with one cacheline access.
>>>>
>>>> If I'm understanding the existing code correctly,
>>>> int dirty_flags can be combined, like VGA + MIGRATION.
>>>> If we only have to worry about a single dirty flag, I agree with 
>>>> your idea.
>>>
>>> From a quick grep it seems flags are not combined, except for 
>>> something strange with CODE_DIRTY_FLAG:
>>>
>>>> static void notdirty_mem_writel(void *opaque, target_phys_addr_t 
>>>> ram_addr,
>>>>                                 uint32_t val)
>>>> {
>>>>     int dirty_flags;
>>>>     dirty_flags = phys_ram_dirty[ram_addr >> TARGET_PAGE_BITS];
>>>>     if (!(dirty_flags & CODE_DIRTY_FLAG)) {
>>>> #if !defined(CONFIG_USER_ONLY)
>>>>         tb_invalidate_phys_page_fast(ram_addr, 4);
>>>>         dirty_flags = phys_ram_dirty[ram_addr >> TARGET_PAGE_BITS];
>>>> #endif
>>>>     }
>>>>     stl_p(qemu_get_ram_ptr(ram_addr), val);
>>>>     dirty_flags |= (0xff & ~CODE_DIRTY_FLAG);
>>>>     phys_ram_dirty[ram_addr >> TARGET_PAGE_BITS] = dirty_flags;
>>>>     /* we remove the notdirty callback only if the code has been
>>>>        flushed */
>>>>     if (dirty_flags == 0xff)
>>>>         tlb_set_dirty(cpu_single_env, cpu_single_env->mem_io_vaddr);
>>>> }
>>>
>>> I can't say I understand what it does.
>>
>> The semantics of CODE_DIRTY_FLAG are a little counter intuitive.  
>> CODE_DIRTY_FLAG means that we know that something isn't code so 
>> writes do not need checking for self modifying code.
>
> So the hardware equivalent is, when the Instruction TLB loads a page 
> address, clear CODE_DIRTY_FLAG?

Yes, and is what tlb_protect_code() does and it's called from 
tb_alloc_page() which is what's code when a TB is created.

Regards,

Anthony Liguori