On Thu, Dec 08, 2022, Ricardo Koller wrote:
On Thu, Dec 08, 2022 at 07:01:57PM +0000, Sean Christopherson wrote:
On Thu, Dec 08, 2022, Ricardo Koller wrote:
On Thu, Dec 08, 2022 at 12:37:23AM +0000, Oliver Upton wrote:
On Thu, Dec 08, 2022 at 12:24:20AM +0000, Sean Christopherson wrote:
Even still, that's just a kludge to make ucalls work. We have other MMIO devices (GIC distributor, for example) that work by chance since nothing conflicts with the constant GPAs we've selected in the tests.
I'd rather we go down the route of having an address allocator for the for both the VA and PA spaces to provide carveouts at runtime.
Aren't those two separate issues? The PA, a.k.a. memslots space, can be solved by allocating a dedicated memslot, i.e. doesn't need a carve. At worst, collisions will yield very explicit asserts, which IMO is better than whatever might go wrong with a carve out.
Perhaps the use of the term 'carveout' wasn't right here.
What I'm suggesting is we cannot rely on KVM memslots alone to act as an allocator for the PA space. KVM can provide devices to the guest that aren't represented as memslots. If we're trying to fix PA allocations anyway, why not make it generic enough to suit the needs of things beyond ucalls?
One extra bit of information: in arm, IO is any access to an address (within bounds) not backed by a memslot. Not the same as x86 where MMIO are writes to read-only memslots. No idea what other arches do.
I don't think that's correct, doesn't this code turn write abort on a RO memslot into an io_mem_abort()? Specifically, the "(write_fault && !writable)" check will match, and assuming none the the edge cases in the if-statement fire, KVM will send the write down io_mem_abort().
You are right. In fact, page_fault_test checks precisely that: writes on RO memslots are sent to userspace as an mmio exit. I was just referring to the MMIO done for ucall.
To clarify for others, Ricardo thought that x86 selftests were already using a read-only memslot for ucalls, hence the confusion.
Having said that, we could use ucall as writes on read-only memslots like what x86 does.
+1. x86 currently uses I/O with a hardcoded port, but theoretically that's just as error prone as hardcoding a GPA, it just works because x86 doesn't have any port I/O tests.
Ugh, and that made me look at sync_regs_test.c, which does its own open coded ucall. That thing is probably working by dumb luck at this point.