Virtio Queries

Virtio Queries

[Kumar, Venkat]

Hi, I am a few questions on Virtio, Hope somebody clarifies them

1. What is the address type that is put inside the vring descriptor in the virt queue while placing a request? Is it the guest virtual address or guest physical address?
2. If it is Guest physical address, how qemu converts it to its virtual address before processing the buffer?
3. How does qemu interact with guest once it receives the buffers from the virt queue for doing the IO?

Thx,

Venkat

Re: Virtio Queries

[Avi Kivity]

Kumar, Venkat wrote:
> Hi, I am a few questions on Virtio, Hope somebody clarifies them
>
> 1. What is the address type that is put inside the vring descriptor in the virt queue while placing a request? Is it the guest virtual address or guest physical address?
>   

Guest physical address.  Guest virtual addresses change meanings from 
time to time.

> 2. If it is Guest physical address, how qemu converts it to its virtual address before processing the buffer?
>   

See the logic in cpu_physical_memory_rw() and cpu_physical_memory_map().

> 3. How does qemu interact with guest once it receives the buffers from the virt queue for doing the IO?
>   

qemu advanced the ring indexes, and notifies the guest by sending an 
interrupt when the requests are completed.

-- 
error compiling committee.c: too many arguments to function

RE: Virtio Queries

[Kumar, Venkat]

> 2. If it is Guest physical address, how qemu converts it to its virtual address before processing the buffer?
>   

See the logic in cpu_physical_memory_rw() and cpu_physical_memory_map().

==> Thanks for the reply. Are these the functions which convert the Guest physical addresses to qemu Virtual addresses? If yes, I did not find any routine calling these functions in the Virtio block IO flow. May be I missed it. Can you please point me to the code where Qemu's backend virtio driver is taking the SGE from the virt ring descriptor after it gets a notification from the guest and converting it to a qemu virtual address before calling "read or write" system calls.

Re: Virtio Queries

[Avi Kivity]

Kumar, Venkat wrote:
>> 2. If it is Guest physical address, how qemu converts it to its virtual address before processing the buffer?
>>   
>>     
>
> See the logic in cpu_physical_memory_rw() and cpu_physical_memory_map().
>
> ==> Thanks for the reply. Are these the functions which convert the Guest physical addresses to qemu Virtual addresses? 

Yes.

> If yes, I did not find any routine calling these functions in the Virtio block IO flow. May be I missed it. Can you please point me to the code where Qemu's backend virtio driver is taking the SGE from the virt ring descriptor after it gets a notification from the guest and converting it to a qemu virtual address before calling "read or write" system calls.
>   

It's in hw/virtio.c, see virtqueue_pop().

virtio-blk.c will issue the actual read or write (via 
bdrv_aio_{read,write}v()).

-- 
error compiling committee.c: too many arguments to function