Re: [PATCH v3 2/7] x86/sgx: Add infrastructure to identify SGX EPC pages

From: Jarkko Sakkinen
Date: Mon Aug 02 2021 - 04:48:50 EST


On Fri, Jul 30, 2021 at 04:46:07PM +0000, Sean Christopherson wrote:
> On Fri, Jul 30, 2021, Jarkko Sakkinen wrote:
> > On Wed, Jul 28, 2021 at 03:19:46PM -0700, Dave Hansen wrote:
> > > On 7/28/21 1:46 PM, Tony Luck wrote:
> > > > Export a function sgx_is_epc_page() that simply reports whether an
> > > > address is an EPC page for use elsewhere in the kernel.
> > >
> > > It would be really nice to mention why this needs to be exported to
> > > modules. I assume it's the error injection driver or something that can
> > > be built as a module, but this export was a surprise when I saw it.
> > >
> > > It's probably also worth noting that this is a sloooooooow
> > > implementation compared to the core VM code that does something
> > > analogous: pfn_to_page(). It's fine for error handling, but we should
> > > probably have a comment to this effect so that more liberal use doesn't
> > > creep in anywhere.
> >
> > You could also create an xarray to track physical EPC address ranges,
> > and make the query fast.
>
> Eh, it's not _that_ slow due to the constraints on the number of EPC sections.
> The hard limit is currently '8', and practically speaking there will be one
> section per socket. Turning a linear search into a binary search in this case
> isn't going to buy much.

Also, consumes more memory.

Just pointing out that it is possible to improve without much fuzz, if ever
required, for instance by using DEFINE_XARRAY() to be define file-scope
xarray.

> Out of curiosity, on multi-socket systems, are EPC sections clustered in a single
> address range, or are they interleaved with regular RAM? If they're clustered,
> you could track the min/max across all sections to optimize the common case that
> an address isn't in any EPC section.

Given that physical address ranges of different NUMA nodes are disjoint,
and each has EPC section is reserved from one such section, I would presume
that they are interleaved.

> static struct sgx_epc_page *sgx_paddr_to_page(u64 paddr)
> {
> struct sgx_epc_section *section;
> int i;
>
> if (paddr < min_epc_pa || paddr > max_epc_pa)
> return NULL;
>
> for (i = 0; i < ARRAY_SIZE(sgx_epc_sections); i++) {
> section = &sgx_epc_sections[i];
>
> if (paddr < section->phys_addr || paddr > section->end_phys_addr)
> continue;
>
> return &section->pages[PFN_DOWN(paddr - section->phys_addr)];
> }
>
> return NULL;
> }

/Jarkko