Re: [PATCH V1 11/16] xen/ioreq: Introduce hvm_domain_has_ioreq_server()

[Julien Grall <julien@xxxxxxx>]

On 16/09/2020 09:13, Paul Durrant wrote:
> > -----Original Message-----
> > From: Jan Beulich <jbeulich@xxxxxxxx>
> > Sent: 16 September 2020 09:05
> > To: Oleksandr Tyshchenko <olekstysh@xxxxxxxxx>; Paul Durrant <paul@xxxxxxx>
> > Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; Oleksandr Tyshchenko 
> > <oleksandr_tyshchenko@xxxxxxxx>; Stefano
> > Stabellini <sstabellini@xxxxxxxxxx>; Julien Grall <julien@xxxxxxx>; Volodymyr 
> > Babchuk
> > <Volodymyr_Babchuk@xxxxxxxx>; Andrew Cooper <andrew.cooper3@xxxxxxxxxx>; Wei Liu 
> > <wl@xxxxxxx>; Roger
> > Pau Monné <roger.pau@xxxxxxxxxx>; Julien Grall <julien.grall@xxxxxxx>
> > Subject: Re: [PATCH V1 11/16] xen/ioreq: Introduce hvm_domain_has_ioreq_server()
> >
> > On 10.09.2020 22:22, Oleksandr Tyshchenko wrote:
> > > From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
> > >
> > > This patch introduces a helper the main purpose of which is to check
> > > if a domain is using IOREQ server(s).
> > >
> > > On Arm the benefit is to avoid calling handle_hvm_io_completion()
> > > (which implies iterating over all possible IOREQ servers anyway)
> > > on every return in leave_hypervisor_to_guest() if there is no active
> > > servers for the particular domain.
> Is this really worth it? The limit on the number of ioreq serves is small... just 8. 
When I suggested this, I failed to realize there was only 8 IOREQ 
servers available. However, I would not be surprised if this increase 
long term as we want to use
> I doubt you'd be able measure the difference.
Bear in mind that entry/exit to the hypervisor is pretty "cheap" on Arm 
compare to x86. So we want to avoid doing extra work if it is not necessary.
Cheers,

--
Julien Grall