On 01.12.2021 12:58, Andre Przywara wrote: > On Tue, 30 Nov 2021 19:13:41 +0000 > Ayan Kumar Halder <ayan.kumar.hal...@xilinx.com> wrote: >> On 30/11/2021 09:49, Andre Przywara wrote: >>> On Mon, 29 Nov 2021 19:16:38 +0000 >>> Ayan Kumar Halder <ayan.kumar.hal...@xilinx.com> wrote: >>>> At the moment, Xen is only handling data abort with valid syndrome (i.e. >>>> ISV=0). Unfortunately, this doesn't cover all the instructions a domain >>>> could use to access MMIO regions. >>>> >>>> For instance, Xilinx baremetal OS will use: >>>> >>>> volatile u32 *LocalAddr = (volatile u32 *)Addr; >>>> *LocalAddr = Value; >>>> >>>> This leave the compiler to decide which store instructions to use. >>> >>> As mentioned in the other email, this is wrong, if this points to MMIO: >>> don't let the compiler do MMIO accesses. If a stage 2 fault isn't in >>> an MMIO area, you should not see traps that you cannot handle already. >>> >>> So I don't think it's a good idea to use that as an example. And since >>> this patch only seems to address this use case, I would doubt its >>> usefulness in general. >> Yes, I should have fixed the comment. >> >> Currently, I am testing with baremetal app which uses inline assembly >> code with post indexing instructions, to access the MMIO. >> >> ATM, I am testing with 32 bit MMIO only. >> >> On the usefulness, I am kind of torn as it is legitimate for post >> indexing instructions to be used in an inline-assembly code for >> accessing MMIO. However, that may not be something commonly seen. > > It is legitimate, but I question the usefulness: for a start it wouldn't > work under today's Xen (or KVM), and I doubt this would be backported. So > you would always require users to use the newest version of the hypervisor. > Also MMIO accesses are slow anyway, so while using post-indexing might be > convenient from an assembly writer's point of view, it doesn't give you > much performance-wise, probably.
Just so it gets mentioned: Smaller code size may also be a consideration here, beyond the more general one about performance. Jan
