Re: [Qemu-devel] Re: KQEMU code organization

[Anthony Liguori <anthony@codemonkey.ws>]

Fabrice Bellard wrote:
>> This is for device emulation.  KVM doesn't use l1_phys_map() for 
>> things like shadow page table accesses.
>>
>> In the device emulation, we're currently using stl_phys() and 
>> friends.  This goes through a full lookup in l1_phys_map.
>>
>> Looking at other devices, some use phys_ram_base + PA and stl_raw() 
>> which is broken but faster.  A few places call 
>> cpu_get_physical_page_desc(), then use phys_ram_base and stl_raw().  
>> This is okay but it still requires at least one l1_phys_map lookup 
>> per operation in the device (packet receive, io notification, etc.).  
>> I don't think that's going to help much because in our fast paths, 
>> we're only doing 2 or 3 stl_phys() operations.
>>
>> At least on x86, there are very few regions of RAM.  That makes it 
>> very easy to cache.  A TLB style cache seems wrong to me because 
>> there are so few RAM regions.  I don't see a better way to do this 
>> with the existing APIs.
>
> I see your point. st/ldx_phys() were never optimized in fact.
>
> A first solution would be to use a cache similar to the TLBs. It has 
> the advantage is being quite generic and fast. Another solution would 
> be to compute a few intervals with are tested before the generic case. 
> These intervals would correspond to the main RAM area and would be 
> updated each time a new device region is registered.

I currently have a patch that takes the later approach.

> Does your remark implies that KVM switches back to the QEMU process 
> for each I/O ? If so, the l1_phys_map access time should be negligible 
> compared to the SVM-VMX/kernel/user context switch !

Most MMIO/PIO cause an exit to QEMU.  We run the main loop in an 
dedicated thread though so packet delivery is handled without forcing a 
VCPU to exit.

Regards,

Anthony Liguori

> Fabrice.
>
>
>