Re: [PATCH v3 02/18] system/memory: Better describe @plen argument of flatview_translate()

[Philippe Mathieu-Daudé <philmd@xxxxxxxxxx>]

On 30/9/25 11:18, Thomas Huth wrote:
> On 30/09/2025 10.31, Philippe Mathieu-Daudé wrote:
> > Hi Thomas,
> >
> > On 30/9/25 10:24, Thomas Huth wrote:
> > > On 30/09/2025 10.21, Philippe Mathieu-Daudé wrote:
> > > > flatview_translate()'s @plen argument is output-only and can be NULL.
> > > >
> > > > When Xen is enabled, only update @plen_out when non-NULL.
> > > >
> > > > Signed-off-by: Philippe Mathieu-Daudé <philmd@xxxxxxxxxx>
> > > > ---
> > > >   include/system/memory.h | 5 +++--
> > > >   system/physmem.c        | 9 +++++----
> > > >   2 files changed, 8 insertions(+), 6 deletions(-)
> > > >
> > > > diff --git a/include/system/memory.h b/include/system/memory.h
> > > > index aa85fc27a10..3e5bf3ef05e 100644
> > > > --- a/include/system/memory.h
> > > > +++ b/include/system/memory.h
> > > > @@ -2992,13 +2992,14 @@ IOMMUTLBEntry 
> > > > address_space_get_iotlb_entry(AddressSpace *as, hwaddr addr,
> > > >    * @addr: address within that address space
> > > >    * @xlat: pointer to address within the returned memory region 
> > > > section's
> > > >    * #MemoryRegion.
> > > > - * @len: pointer to length
> > > > + * @plen_out: pointer to valid read/write length of the translated 
> > > > address.
> > > > + *            It can be @NULL when we don't care about it.
> > > >    * @is_write: indicates the transfer direction
> > > >    * @attrs: memory attributes
> > > >    */
> > > >   MemoryRegion *flatview_translate(FlatView *fv,
> > > >                                    hwaddr addr, hwaddr *xlat,
> > > > -                                 hwaddr *len, bool is_write,
> > > > +                                 hwaddr *plen_out, bool is_write,
> > > >                                    MemTxAttrs attrs);
> > > >   static inline MemoryRegion *address_space_translate(AddressSpace *as,
> > > > diff --git a/system/physmem.c b/system/physmem.c
> > > > index 8a8be3a80e2..86422f294e2 100644
> > > > --- a/system/physmem.c
> > > > +++ b/system/physmem.c
> > > > @@ -566,7 +566,7 @@ iotlb_fail:
> > > >   /* Called from RCU critical section */
> > > >   MemoryRegion *flatview_translate(FlatView *fv, hwaddr addr, hwaddr 
> > > > *xlat,
> > > > -                                 hwaddr *plen, bool is_write,
> > > > +                                 hwaddr *plen_out, bool is_write,
> > > >                                    MemTxAttrs attrs)
> > > >   {
> > > >       MemoryRegion *mr;
> > > > @@ -574,13 +574,14 @@ MemoryRegion *flatview_translate(FlatView *fv, 
> > > > hwaddr addr, hwaddr *xlat,
> > > >       AddressSpace *as = NULL;
> > > >       /* This can be MMIO, so setup MMIO bit. */
> > > > -    section = flatview_do_translate(fv, addr, xlat, plen, NULL,
> > > > +    section = flatview_do_translate(fv, addr, xlat, plen_out, NULL,
> > > >                                       is_write, true, &as, attrs);
> > > >       mr = section.mr;
> > > > -    if (xen_enabled() && memory_access_is_direct(mr, is_write, 
> > > > attrs)) {
> > > > +    if (xen_enabled() && plen_out && memory_access_is_direct(mr, 
> > > > is_write,
> > > > +                                                             attrs)) {
> > > >           hwaddr page = ((addr & TARGET_PAGE_MASK) + 
> > > > TARGET_PAGE_SIZE) - addr;
> > > > -        *plen = MIN(page, *plen);
> > > > +        *plen_out = MIN(page, *plen_out);
> > > >       }
> > > My question from the previous version is still unanswered:
> > >
> > > https://lore.kernel.org/qemu- 
> > > devel/22ff756a-51a2-43f4-8fe1-05f17ff4a371@xxxxxxxxxx/
> > This patches
> > - checks for plen not being NULL
> > - describes it as
> >    "When Xen is enabled, only update @plen_out when non-NULL."
> > - mention that in the updated flatview_translate() documentation
> >    "It can be @NULL when we don't care about it." as documented for
> >    the flatview_do_translate() callee in commit d5e5fafd11b ("exec:
> >    add page_mask for flatview_do_translate")
> >
> > before:
> >
> >    it was not clear whether we can pass plen=NULL without having
> >    to look at the code.
> >
> > after:
> >
> >    no change when plen is not NULL, we can pass plen=NULL safely
> >    (it is documented).
> >
> > I shouldn't be understanding your original question, do you mind
> > rewording it?
> Ah, you've updated the patch in v3 to include a check for plen_out in 
> the if-statement! It was not there in v2. Ok, this should be fine now:
> Reviewed-by: Thomas Huth <thuth@xxxxxxxxxx>
>
> I just re-complained since you did not respond to my mail in v2, and 
> when I looked at the changelog in your v3 cover letter, you did not 
> mention the modification here, so I blindly assumed that this patch was 
> unchanged.
Ah I see... OK I'll try to be more explicit in my respins.

Thanks for your review!

Phil.