RE: [PATCH V4 08/10] xen/arm: introduce acquire_staticmem_pages and acquire_domstatic_pages

[Penny Zheng <Penny.Zheng@arm.com>]

> -----Original Message-----
> From: Julien Grall <julien@xen.org>
> Sent: Tuesday, August 17, 2021 1:44 AM
> To: Penny Zheng <Penny.Zheng@arm.com>; xen-devel@lists.xenproject.org;
> sstabellini@kernel.org
> Cc: Bertrand Marquis <Bertrand.Marquis@arm.com>; Wei Chen
> <Wei.Chen@arm.com>; nd <nd@arm.com>
> Subject: Re: [PATCH V4 08/10] xen/arm: introduce acquire_staticmem_pages
> and acquire_domstatic_pages
> 
> 
> 
> On 16/08/2021 07:43, Penny Zheng wrote:
> > Hi Julien,
> 
> Hi,
> 
> >>> +{
> >>> +    bool need_tlbflush = false;
> >>> +    uint32_t tlbflush_timestamp = 0;
> >>> +    unsigned long i;
> >>> +    struct page_info *pg;
> >>> +
> >>> +    /* For now, it only supports pre-configured static memory. */
> >>
> >> This comment doesn't seem to match the check below.
> >>
> >>> +    if ( !mfn_valid(smfn) || !nr_mfns )
> >>
> >> This check only guarantees that there will be a page for the first MFN.
> >> Shouldn't we also check for the other MFNs?
> >>
> >
> > Hmm, Do you think that it should be all checked, the whole range, [smfn,
> smfn + nr_mfns).
> > Since it is in linear growth, maybe adding another check of "!mfn_valid(smfn
> + nr_mfns - 1)"
> > is enough?
> 
> The only requirement for Xen is to provide a valid struct page for each RAM
> page. So checking the first and end page may not be sufficient if there is a hole
> in the middle.
> 
> My point here is either:
>    - we trust the callers so we remove the mfn_valid() check
>    - we don't trust the callers so we check the MFN is valid for every page
> 
> My preference is the latter, the more if we plan to us the helper after boot in
> the future. An possible compromise is to add a comment that this function
> needs to be reworked if used outside of boot.
> 

Ok. I'll do the whole range check and add the comments.

> Cheers,
> 
> 
> --
> Julien Grall