On Jan 8, 2016 3:41 PM, "Linus Torvalds" [off-list ref] wrote:

On Fri, Jan 8, 2016 at 3:15 PM, Andy Lutomirski [off-list ref] wrote:

quoted

+       /*
+        * We mustn't be preempted or handle an IPI while reading and
+        * writing CR3.  Preemption could switch mms and switch back, and
+        * an IPI could call leave_mm.  Either of those could cause our
+        * PCID to change asynchronously.
+        */
+       raw_local_irq_save(flags);
        native_write_cr3(native_read_cr3());
+       raw_local_irq_restore(flags);

This seems sad for two reasons:

 - it adds unnecessary overhead on non-pcid setups (32-bit being an
example of that)

I can certainly skip the flag saving on !PCID.

 - on pcid setups, wouldn't invpcid_flush_single_context() be better?

I played with that and it was slower.  I don't pretend that makes any sense.

So on the whole I hate it.

Why isn't this something like

        if (static_cpu_has_safe(X86_FEATURE_INVPCID)) {
                invpcid_flush_single_context();
                return;
        }
        native_write_cr3(native_read_cr3());

*without* any flag saving crud?

And yes, that means that we'd require X86_FEATURE_INVPCID in order to
use X86_FEATURE_PCID, but that seems fine.

I have an SNB "Extreme" with PCID but not INVPCID, and there could be
a whole generation of servers like that.  I think we should fully
support them.

We might be able to get away with just disabling preemption instead of
IRQs, at least if mm == active_mm.

Or is there some reason you wanted the odd flags version? If so, that
should be documented.

What do you mean "odd"?

--Andy

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