On Tue, 2018-05-22 at 17:47 +0200, Paolo Bonzini wrote: > On 22/05/2018 17:42, Raslan, KarimAllah wrote: > > > > On Mon, 2018-04-16 at 18:28 +0200, Paolo Bonzini wrote: > > > > > > On 15/04/2018 00:26, KarimAllah Ahmed wrote: > > > > > > > > > > > > Switch 'requests' to be explicitly 64-bit and update BUILD_BUG_ON check > > > > to > > > > use the size of "requests" instead of the hard-coded '32'. > > > > > > > > That gives us a bit more room again for arch-specific requests as we > > > > already ran out of space for x86 due to the hard-coded check. > > > > > > > > Cc: Paolo Bonzini <pbonz...@redhat.com> > > > > Cc: Radim Krčmář <rkrc...@redhat.com> > > > > Cc: k...@vger.kernel.org > > > > Cc: linux-kernel@vger.kernel.org > > > > Signed-off-by: KarimAllah Ahmed <karah...@amazon.de> > > > > > > I'm afraid architectures like ARM 32 need this to be conditional (using > > > Kconfig). > > > > Why would using a 64-bit 'requests' be a problem for ARM32? Are you > > concerned about performance here or is there some symantic problem? > > They don't support atomics on double-word data.
But they support atomics on single words. Of which there are two. We don't need atomic updates of the whole 64-bit quantity (a là cmpxchg). Do we strictly need this to be atomic accross the 64-bit? Looking at the use cases for "requests": kvm_clear_request kvm_test_request kvm_request_pending kvm_check_request ... and all of them would still work if the atomicity is only at the word level, right? > > Paolo > Amazon Development Center Germany GmbH Berlin - Dresden - Aachen main office: Krausenstr. 38, 10117 Berlin Geschaeftsfuehrer: Dr. Ralf Herbrich, Christian Schlaeger Ust-ID: DE289237879 Eingetragen am Amtsgericht Charlottenburg HRB 149173 B
