Hi Andrea,

On Thu, Feb 1, 2024 at 4:03 PM Andrea Parri [off-list ref] wrote:

On Wed, Jan 31, 2024 at 04:59:29PM +0100, Alexandre Ghiti wrote:

quoted

The preventive sfence.vma were emitted because new mappings must be made
visible to the page table walker but Svvptc guarantees that xRET act as
a fence, so no need to sfence.vma for the uarchs that implement this
extension.

AFAIU, your first submission shows that you don't need that xRET property.
Similarly for other archs.  What was rationale behind this Svvptc change?

Actually, the ARC has just changed its mind and removed this new
behaviour from the Svvptc extension, so we will take some gratuitous
page faults (but that should be outliners), which makes riscv similar
to x86 and arm64.

quoted

This allows to drastically reduce the number of sfence.vma emitted:

* Ubuntu boot to login:
Before: ~630k sfence.vma
After:  ~200k sfence.vma

* ltp - mmapstress01
Before: ~45k
After:  ~6.3k

* lmbench - lat_pagefault
Before: ~665k
After:   832 (!)

* lmbench - lat_mmap
Before: ~546k
After:   718 (!)

This Svvptc seems to move/add the "burden" of the synchronization to xRET:
Perhaps integrate the above counts w/ the perf gains in the cover letter?

Yes, I'll copy that to the cover letter.

Thanks for your interest!

Alex

  Andrea