Re: [PATCH v7 7/8] xen/riscv: page table handling

[oleksii.kurochko@xxxxxxxxx]

On Wed, 2024-09-25 at 16:58 +0200, Jan Beulich wrote:
> On 25.09.2024 16:50, oleksii.kurochko@xxxxxxxxx wrote:
> > On Wed, 2024-09-25 at 16:22 +0200, Jan Beulich wrote:
> > > On 25.09.2024 12:07, oleksii.kurochko@xxxxxxxxx wrote:
> > > > On Tue, 2024-09-24 at 15:31 +0200, Jan Beulich wrote:
> > > > > On 24.09.2024 13:30, oleksii.kurochko@xxxxxxxxx wrote:
> > > > > > On Tue, 2024-09-24 at 12:49 +0200, Jan Beulich wrote:
> > > > > > > On 13.09.2024 17:57, Oleksii Kurochko wrote:
> > > > > > > > +static int pt_next_level(bool alloc_tbl, pte_t
> > > > > > > > **table,
> > > > > > > > unsigned
> > > > > > > > int offset)
> > > > > > > > +{
> > > > > > > > +    pte_t *entry;
> > > > > > > > +    mfn_t mfn;
> > > > > > > > +
> > > > > > > > +    entry = *table + offset;
> > > > > > > > +
> > > > > > > > +    if ( !pte_is_valid(*entry) )
> > > > > > > > +    {
> > > > > > > > +        if ( !alloc_tbl )
> > > > > > > > +            return XEN_TABLE_MAP_FAILED;
> > > > > > > > +
> > > > > > > > +        if ( create_table(entry) )
> > > > > > > > +            return XEN_TABLE_MAP_FAILED;
> > > > > > > 
> > > > > > > You're still losing the -ENOMEM here.
> > > > > > Agree, I will save the return value of create_table and
> > > > > > return
> > > > > > it.
> > > > > 
> > > > > That won't work very well, will it? 
> > > > I think it will work, just will be needed another one check in
> > > > pt_update_entry() where pt_next_level() is called:
> > > >         if ( (rc == XEN_TABLE_MAP_FAILED) || (rc == -ENOMEM) )
> > > >         ...
> > > 
> > > Yet that's precisely why I said "won't work very well": You're
> > > now
> > > having
> > > rc in two entirely distinct number spaces (XEN_TABLE_MAP_* and -
> > > E*).
> > > That's imo just calling for trouble down the road. Unless you
> > > emphasized
> > > this aspect pretty well in a comment.
> > > 
> > > > > Imo you need a new XEN_TABLE_MAP_NOMEM.
> > > > > (And then XEN_TABLE_MAP_FAILED may want renaming to e.g.
> > > > > XEN_TABLE_MAP_NONE).
> > > > I am still curious if we really need this separation. If to in
> > > > this
> > > > way
> > > > then it should be updated the check in pt_update_entry():
> > > >    --- a/xen/arch/riscv/pt.c
> > > >    +++ b/xen/arch/riscv/pt.c
> > > >    @@ -165,10 +165,10 @@ static int pt_next_level(bool
> > > > alloc_tbl,
> > > > pte_t
> > > >    **table, unsigned int offset)
> > > >         if ( !pte_is_valid(*entry) )
> > > >         {
> > > >             if ( !alloc_tbl )
> > > >    -            return XEN_TABLE_MAP_FAILED;
> > > >    +            return XEN_TABLE_MAP_NONE;
> > > >     
> > > >             if ( create_table(entry) )
> > > >    -            return XEN_TABLE_MAP_FAILED;
> > > >    +            return XEN_TABLE_MAP_NOMEM;
> > > >         }
> > > >     
> > > >         if ( pte_is_mapping(*entry) )
> > > >    @@ -209,7 +209,7 @@ static int pt_update_entry(mfn_t root,
> > > > unsigned
> > > >    long virt,
> > > >         for ( ; level > target; level-- )
> > > >         {
> > > >             rc = pt_next_level(alloc_tbl, &table,
> > > > offsets[level]);
> > > >    -        if ( rc == XEN_TABLE_MAP_FAILED )
> > > >    +        if ( (rc == XEN_TABLE_MAP_NONE) && (rc ==
> > > > XEN_TABLE_MAP_NOMEM)
> > > >    )
> > > >             {
> > > >                 rc = 0;
> > > > But the handling of XEN_TABLE_MAP_NONE and XEN_TABLE_MAP_NOMEM
> > > > seems to
> > > > me should be left the same as this one part of the code
> > > > actually
> > > > catching the case when create_table() returns -ENOMEM:
> > > >    pt_next_level()
> > > >    {
> > > >    ...
> > > >                if ( flags & (PTE_VALID | PTE_POPULATE) )
> > > >                {
> > > >                    dprintk(XENLOG_ERR,
> > > >                            "%s: Unable to map level %u\n",
> > > > __func__,
> > > >    level);
> > > >                    rc = -ENOMEM;
> > > >                }
> > > 
> > > Except that you want to avoid "inventing" an error code when you
> > > were
> > > handed one. Just consider what happens to this code if another -
> > > E...
> > > could also come back from the helper.
> > I think we can drop the usage of -ENOMEM in the helper
> > create_table()
> > by returning XEN_TABLE_MAP_FAILED in case of failure, with a
> > redefinition of XEN_TABLE_MAP_FAILED = 1, XEN_TABLE_SUPER_PAGE = 2,
> > and
> > XEN_TABLE_NORMAL = 3, as value 0 is used to indicate that
> > everything is
> > okay.
> > 
> > We can leave the pt_update() code as it is now:
> > ...
> >             if ( flags & (PTE_VALID | PTE_POPULATE) )
> >             {
> >                 dprintk(XENLOG_ERR,
> >                       "%s: Unable to map level %u\n", __func__,
> > level);
> >                 rc = -ENOMEM;
> >             }
> > ...
> > 
> > Because for the end user, it's better to receive the error code
> > from
> > xen/errno.h rather than a custom error code introduced nearby the
> > helper.
> > 
> > Does it make sense?
> 
> While I think I see where you're coming from, I still don't agree.
> And
> I never suggested to bubble up some custom error indication. Up the
> call
> chain it wants to be -ENOMEM, sure. Yet keying its generation to
> flags & (PTE_VALID | PTE_POPULATE) is both less obvious and more
> fragile
> (towards future code changes) than keying it to rc ==
> XEN_TABLE_MAP_NOMEM.
I am not sure that (rc == XEN_TABLE_MAP_NOMEM) is equal to (flags &
(PTE_VALID | PTE_POPULATE) as XEN_TABLE_MAP_NOMEM miss the case (flags
& PTE_VALID) == 0 ( removing a mapping case ) and for which should be
returned 0 but not -ENOMEM.

~ Oleksii

> 
> If I can't convince you, then so be it. Yet then I'm also not very
> likely
> to ack this patch of yours (which of course won't mean I wouldn't
> further
> look at other aspects of the change, hopefully at least making it
> easier
> for someone else to ack it).