amd64: even up copyin/copyout with memcpy + other cleanup

[mjg]

Summary

• _fault handlers for both primitives are identical, provide just one
• change the copying scheme to match memcpy (in particular jump avoidance for the most common case of multiply of 8)
• stop re-reading pcb address on exit, just store it locally (in r9)

This last bit can be applied to all other primitives as well. Side point is that there is a way to get rid of the need of setting onfault handler by looking up RIP at page fault and comparing it against userspace access routines. I have a prototype which encloses rep sections with labels for identification.

Regardless of the above there is more cleanup to be done here.

[kib]

Not setting pcb_onfault has more consequences than only comparing addresses in trap(). But I will comment more on the actual patch.

sys/amd64/amd64/support.S

413 ↗

(On Diff #48287)

This should be jbe and not jle (signed vs unsigned), I believe.

458 ↗

(On Diff #48287)

This can be movl $EFAULT,%eax.

[mjg]

In D17265#368054, @kib wrote:

Not setting pcb_onfault has more consequences than only comparing addresses in trap(). But I will comment more on the actual patch.

If you mean it needs a place to land to handle recovery, this is covered with a table:

CNAME(onfault_table_nosmap):
       .quad ot_copyout_nosmap_start
       .quad ot_copyout_nosmap_end
       .quad copy_fault
       .quad ot_copyin_nosmap_start
       .quad ot_copyin_nosmap_end
       .quad copy_fault
       .quad 0

I'm mostly toying with this idea. I have a working prototype which I'll possibly propose after 12 ships. I don't know where the general idea originates from, NetBSD and Linux have a variant.

[kib]

In D17265#368078, @mjg wrote:

In D17265#368054, @kib wrote:

Not setting pcb_onfault has more consequences than only comparing addresses in trap(). But I will comment more on the actual patch.

If you mean it needs a place to land to handle recovery, this is covered with a table:

CNAME(onfault_table_nosmap):
       .quad ot_copyout_nosmap_start
       .quad ot_copyout_nosmap_end
       .quad copy_fault
       .quad ot_copyin_nosmap_start
       .quad ot_copyin_nosmap_end
       .quad copy_fault
       .quad 0

I'm mostly toying with this idea. I have a working prototype which I'll possibly propose after 12 ships. I don't know where the general idea originates from, NetBSD and Linux have a variant.

I mean that there are more places which make a decision based on pcb_onfault != 0 than just a fault landing code. Grep for pcb_onfault.