On Wed, Jan 17, 2024 at 12:30:00PM +0000, Mark Rutland wrote:
> On Tue, Jan 16, 2024 at 02:51:21PM -0400, Jason Gunthorpe wrote:
> > Hey Catalin,
> >
> > I'm just revising this and I'm wondering if you know why ARM64 has this:
> >
> > #define __raw_writeq __raw_writeq
> > static __always_inline void __raw_writeq(u64 val, volatile void __iomem *addr)
> > {
> > asm volatile("str %x0, [%1]" : : "rZ" (val), "r" (addr));
> > }
> >
> > Instead of
> >
> > #define __raw_writeq __raw_writeq
> > static __always_inline void __raw_writeq(u64 val, volatile void __iomem *addr)
> > {
> > asm volatile("str %x0, %1" : : "rZ" (val), "m" (*(volatile u64 *)addr));
> > }
> >
> > ?? Like x86 has.
>
> I believe this is for the same reason as doing so in all of our other IO
> accessors.
>
> We've deliberately ensured that our IO accessors use a single base register
> with no offset as this is the only form that HW can represent in ESR_ELx.ISS.SRT
> when reporting a stage-2 abort, which a hypervisor may use for emulating IO.
FWIW, IIUC the immediate-offset forms *without* writeback can still be reported
usefully in ESR_ELx, so I believe that we could use the "o" constraint for the
__raw_write*() functions, e.g.
static __always_inline void __raw_writeq(u64 val, volatile void __iomem *addr)
{
asm volatile("str %x0, %1" : : "rZ" (val), "o" (*(volatile u64 *)addr));
}
However, the __raw_read*() functions would still need to use "r" due to
ARM64_WORKAROUND_DEVICE_LOAD_ACQUIRE.
Mark.
