Re: [PATCH V4 14/24] arm/ioreq: Introduce arch specific bits for IOREQ/DM features

[Julien Grall <julien@xxxxxxx>]

Hi Oleksandr,

On 17/01/2021 18:52, Oleksandr wrote:
> On 17.01.21 20:07, Julien Grall wrote:
> > On 17/01/2021 17:11, Oleksandr wrote:
> > > On 15.01.21 22:26, Julien Grall wrote:
> > >
> > > Hi Julien
> > Hi Oleksandr,
>
> Hi Julien
>
>
>
> > > > > +
> > > > >       PROGRESS(xen):
> > > > >           ret = relinquish_memory(d, &d->xenpage_list);
> > > > >           if ( ret )
> > > > > diff --git a/xen/arch/arm/io.c b/xen/arch/arm/io.c
> > > > > index ae7ef96..9814481 100644
> > > > > --- a/xen/arch/arm/io.c
> > > > > +++ b/xen/arch/arm/io.c
> > > > > @@ -16,6 +16,7 @@
> > > > >    * GNU General Public License for more details.
> > > > >    */
> > > > >   +#include <xen/ioreq.h>
> > > > >   #include <xen/lib.h>
> > > > >   #include <xen/spinlock.h>
> > > > >   #include <xen/sched.h>
> > > > > @@ -23,6 +24,7 @@
> > > > >   #include <asm/cpuerrata.h>
> > > > >   #include <asm/current.h>
> > > > >   #include <asm/mmio.h>
> > > > > +#include <asm/hvm/ioreq.h>
> > > > Shouldn't this have been included by "xen/ioreq.h"?
> > > Well, for V1 asm/hvm/ioreq.h was included by xen/ioreq.h. But, it 
> > > turned out that there was nothing inside common header required arch 
> > > one to be included and
> > > I was asked to include arch header where it was indeed needed 
> > > (several *.c files).
> > Fair enough.
> >
> > [...]
> >
> > > > If you return IO_HANDLED here, then it means the we will take care 
> > > > of previous I/O but the current one is going to be ignored. 
> > > Which current one? As I understand, if try_fwd_ioserv() gets called 
> > > with vio->req.state == STATE_IORESP_READY then this is a second round 
> > > after emulator completes the emulation (the first round was when
> > > we returned IO_RETRY down the function and claimed that we would need 
> > > a completion), so we are still dealing with previous I/O.
> > > vcpu_ioreq_handle_completion() -> arch_ioreq_complete_mmio() -> 
> > > try_handle_mmio() -> try_fwd_ioserv() -> handle_ioserv()
> > > And after we return IO_HANDLED here, handle_ioserv() will be called 
> > > to complete the handling of this previous I/O emulation.
> > > Or I really missed something?
> > Hmmm... I somehow thought try_fw_ioserv() would only be called the 
> > first time. Do you have a branch with your code applied? This would 
> > help to follow the different paths.
> Yes, I mentioned about it in cover letter.
>
> Please see
> https://github.com/otyshchenko1/xen/commits/ioreq_4.14_ml5
> why 5 - because I started counting from the RFC)
Oh, I looked at the cover letter and didn't find it. Hence why I asked. 
I should have looked more carefully. Thanks!
I have looked closer at the question and I am not sure to understand why 
arch_ioreq_complete_mmio() is going to call try_handle_mmio().
This looks pretty innefficient to me because we already now the IO was 
handled by the IOREQ server.
I realize that x86 is calling handle_mmio() again. However, I don't 
think we need the same on Arm because the instruction for accessing 
device memory are a lot simpler (you can only read or store at most a 
64-bit value).
So I would like to keep our emulation simple and not rely on 
try_ioserv_fw() to always return true when call from completion (AFAICT 
it is not possible to return false then).
I will answer to the rest separately.

Cheers,

--
Julien Grall