Re: [PATCH v4] x86/sgx: Free backing memory after faulting the enclave page

[Jarkko Sakkinen <jarkko@kernel.org>]

On Thu, Feb 24, 2022 at 09:14:05AM -0800, Dave Hansen wrote:
> On 2/22/22 04:03, Jarkko Sakkinen wrote:
> > +	if (pcmd_page_empty) {
> > +		pgoff_t pcmd_off = encl->size + PAGE_SIZE /* SECS */ +
> > +				   page_index * sizeof(struct sgx_pcmd);
> > +
> > +		sgx_encl_truncate_backing_page(encl, PFN_DOWN(pcmd_off));
> > +	}
> > +
> >  	return ret;
> >  }
> >  
> > @@ -583,7 +613,7 @@ static struct page *sgx_encl_get_backing_page(struct sgx_encl *encl,
> >  static int sgx_encl_get_backing(struct sgx_encl *encl, unsigned long page_index,
> >  				struct sgx_backing *backing)
> >  {
> > -	pgoff_t pcmd_index = PFN_DOWN(encl->size) + 1 + (page_index >> 5);
> > +	pgoff_t pcmd_off = encl->size + PAGE_SIZE /* SECS */ + page_index * sizeof(struct sgx_pcmd);
> 
> Jarkko, I really don't like how this looks.  The '/* SECS */' thing is
> pretty ugly and the comment in the middle of an arithmetic operation is
> just really hard to read.
> 
> Then, there's the fact that this gem is copied-and-pasted.  Oh, and it
> looks a wee bit over 80 columns.

Today you can have 100.

> 
> I went to the trouble of writing a nice, fully-fleshed-out helper
> function for this with a comment included:
> 
> > https://lore.kernel.org/all/8afec431-4dfc-d8df-152b-76cca0e17ccb@intel.com/

Keeping full byte offset up until parts of it are required for something
makes the formula just a simple equation of additions and multiplications,
e.g. nothing like "/ sizeof(struct sgx_pcmd)" is required.

Then you get the PCMD page index will be just PFN_DOWN(pcmd_off) and offset
inside that page is pcmd_off & PAGE_MASK. At least fro me this is more 
intuitive way to do the calculations.
 
I thought that the formula is so simple that it does not matter if it is
just in two sites open coded but I can wrap it too, if required, e.g.

/* 
 * Calculate byte offset of a PCMD struct associated to an enclave page.
 * PCMD's follow right after the EPC data in the backing storage. In
 * addition to the visible enclave pages, there's one extra page slot
 * for SECS, before PCMD data.
 */
static pgoff_t *sgx_encl_page_index_to_pcmd_offset(struct sgx_encl *encl, unsigned long page_index)
{
        return encl->size + PAGE_SIZE + page_index * sizeof(struct sgx_pcmd);
}

> 
> Was there a problem using that?  The change from the last version is:
> 
> * Sanitized the offset calculations.
> 
> Given that there have been multiple different calculations over the four
> versions so far, which version was right?  v3 or v4?

This one has correct and tested calculations but for peer test probably
Reinette should verify that. I tested this with my laptop in bare metal.

BR, Jarkko