[PATCH rdma-next 1/2] arm64/io: add memcpy_toio_64

[Jason Gunthorpe]

On Wed, Jan 17, 2024 at 08:36:18AM -0400, Jason Gunthorpe wrote:
> 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.
> 
> Wow, harming bare metal performace to accommodate imperfect emulation
> sounds like a horrible reason :(
> 
> So what happens with this patch where IO is done with STP? Are you
> going to tell me I can't do it because of this?

I should also point out that userspace IO doesn't follow such a
restriction since nobody ever knew, and things like RDMA stack and
DPDK use ST4 and probably other non-trivial instructions for IO from
userspace.

I'm fearful you saying today's hypervisors don't work right on ARM if
IO is trapped, which does happen for some legimiate reasons and a few
illegitimate ones??

Jason