Re: [PATCH V2 17/23] xen/ioreq: Introduce domain_has_ioreq_server()

[Oleksandr <olekstysh@xxxxxxxxx>]

On 20.10.20 13:51, Paul Durrant wrote:

Hi Paul.

Sorry for the late response.

> > -----Original Message-----
> > From: Oleksandr Tyshchenko <olekstysh@xxxxxxxxx>
> > Sent: 15 October 2020 17:44
> > To: xen-devel@xxxxxxxxxxxxxxxxxxxx
> > Cc: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>; Stefano Stabellini 
> > <sstabellini@xxxxxxxxxx>;
> > Julien Grall <julien@xxxxxxx>; Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>; 
> > Andrew Cooper
> > <andrew.cooper3@xxxxxxxxxx>; George Dunlap <george.dunlap@xxxxxxxxxx>; Ian 
> > Jackson
> > <iwj@xxxxxxxxxxxxxx>; Jan Beulich <jbeulich@xxxxxxxx>; Wei Liu <wl@xxxxxxx>; 
> > Paul Durrant
> > <paul@xxxxxxx>; Julien Grall <julien.grall@xxxxxxx>
> > Subject: [PATCH V2 17/23] xen/ioreq: Introduce domain_has_ioreq_server()
> >
> > 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 current benefit is to avoid calling handle_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.
> > Also this helper will be used by one of the subsequent patches on Arm.
> >
> > This involves adding an extra per-domain variable to store the count
> > of servers in use.
> >
> > Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
> > CC: Julien Grall <julien.grall@xxxxxxx>
> >
> > ---
> > Please note, this is a split/cleanup/hardening of Julien's PoC:
> > "Add support for Guest IO forwarding to a device emulator"
> >
> > Changes RFC -> V1:
> >     - new patch
> >
> > Changes V1 -> V2:
> >     - update patch description
> >     - guard helper with CONFIG_IOREQ_SERVER
> >     - remove "hvm" prefix
> >     - modify helper to just return d->arch.hvm.ioreq_server.nr_servers
> >     - put suitable ASSERT()s
> >     - use ASSERT(d->ioreq_server.server[id] ? !s : !!s) in set_ioreq_server()
> >     - remove d->ioreq_server.nr_servers = 0 from hvm_ioreq_init()
> > ---
> >   xen/arch/arm/traps.c    | 15 +++++++++------
> >   xen/common/ioreq.c      |  7 ++++++-
> >   xen/include/xen/ioreq.h | 14 ++++++++++++++
> >   xen/include/xen/sched.h |  1 +
> >   4 files changed, 30 insertions(+), 7 deletions(-)
> >
> > diff --git a/xen/arch/arm/traps.c b/xen/arch/arm/traps.c
> > index 507c095..a8f5fdf 100644
> > --- a/xen/arch/arm/traps.c
> > +++ b/xen/arch/arm/traps.c
> > @@ -2261,14 +2261,17 @@ static bool check_for_vcpu_work(void)
> >       struct vcpu *v = current;
> >
> >   #ifdef CONFIG_IOREQ_SERVER
> > -    bool handled;
> > +    if ( domain_has_ioreq_server(v->domain) )
> > +    {
> > +        bool handled;
> >
> > -    local_irq_enable();
> > -    handled = handle_io_completion(v);
> > -    local_irq_disable();
> > +        local_irq_enable();
> > +        handled = handle_io_completion(v);
> > +        local_irq_disable();
> >
> > -    if ( !handled )
> > -        return true;
> > +        if ( !handled )
> > +            return true;
> > +    }
> >   #endif
> >
> >       if ( likely(!v->arch.need_flush_to_ram) )
> > diff --git a/xen/common/ioreq.c b/xen/common/ioreq.c
> > index bcd4961..a72bc0e 100644
> > --- a/xen/common/ioreq.c
> > +++ b/xen/common/ioreq.c
> > @@ -39,9 +39,14 @@ static void set_ioreq_server(struct domain *d, unsigned int 
> > id,
> >                                struct ioreq_server *s)
> >   {
> >       ASSERT(id < MAX_NR_IOREQ_SERVERS);
> > -    ASSERT(!s || !d->ioreq_server.server[id]);
> > +    ASSERT(d->ioreq_server.server[id] ? !s : !!s);
> That looks odd. How about ASSERT(!s ^ !d->ioreq_server.server[id])?
ok, looks like it will work.


>    Paul
>
> >       d->ioreq_server.server[id] = s;
> > +
> > +    if ( s )
> > +        d->ioreq_server.nr_servers++;
> > +    else
> > +        d->ioreq_server.nr_servers--;
> >   }
> >
> >   #define GET_IOREQ_SERVER(d, id) \
> > diff --git a/xen/include/xen/ioreq.h b/xen/include/xen/ioreq.h
> > index 7b03ab5..0679fef 100644
> > --- a/xen/include/xen/ioreq.h
> > +++ b/xen/include/xen/ioreq.h
> > @@ -55,6 +55,20 @@ struct ioreq_server {
> >       uint8_t                bufioreq_handling;
> >   };
> >
> > +#ifdef CONFIG_IOREQ_SERVER
> > +static inline bool domain_has_ioreq_server(const struct domain *d)
> > +{
> > +    ASSERT((current->domain == d) || atomic_read(&d->pause_count));
> > +
> This seems like an odd place to put such an assertion.
I might miss something or interpreted incorrectly but these asserts are 
the result of how I understood the review comment on previous version [1].
I will copy a comment here for the convenience:
"This is safe only when d == current->domain and it's not paused,
or when they're distinct and d is paused. Otherwise the result is
stale before the caller can inspect it. This wants documenting by
at least a comment, but perhaps better by suitable ASSERT()s."


> > +    return d->ioreq_server.nr_servers;
> > +}
> > +#else
> > +static inline bool domain_has_ioreq_server(const struct domain *d)
> > +{
> > +    return false;
> > +}
> > +#endif
> > +
> Can this be any more compact? E.g.
>
> return IS_ENABLED(CONFIG_IOREQ_SERVER) && d->ioreq_server.nr_servers;
>
> ?
I have got a compilation error this way (if CONFIG_IOREQ_SERVER is 
disabled):
...xen/4.14.0+gitAUTOINC+ee22110219-r0/git/xen/include/xen/ioreq.h:62:48: 
error: ‘const struct domain’ has no member named ‘ioreq_server’
     return IS_ENABLED(CONFIG_IOREQ_SERVER) && d->ioreq_server.nr_servers;
                                                ^
as domain's ioreq_server struct is guarded by CONFIG_IOREQ_SERVER as well.


[1] 
https://patchwork.kernel.org/project/xen-devel/patch/1599769330-17656-12-git-send-email-olekstysh@xxxxxxxxx/#23618623
Thank you.

--
Regards,

Oleksandr Tyshchenko