[PATCH 10/17] prmem: documentation

[Peter Zijlstra]

On Tue, Oct 30, 2018 at 02:02:12PM -0700, Andy Lutomirski wrote:
> But I dislike allowing regular writes in the protected region. We
> really only need four write primitives:
> 
> 1. Just write one value.  Call at any time (except NMI).
> 
> 2. Just copy some bytes. Same as (1) but any number of bytes.

Given the !preempt/!IRQ contraints I'd certainly put an upper limit on
the number of bytes there.

> 3,4: Same as 1 and 2 but must be called inside a special rare write
> region. This is purely an optimization.
> 
> Actually getting a modifiable pointer should be disallowed for two
> reasons:
> 
> 1. Some architectures may want to use a special
> write-different-address-space operation.

You're thinking of s390 ? :-)

> Heck, x86 could, too: make
> the actual offset be a secret and shove the offset into FSBASE or
> similar. Then %fs-prefixed writes would do the rare writes.

> 2. Alternatively, x86 could set the U bit. Then the actual writes
> would use the uaccess helpers, giving extra protection via SMAP.

Cute, and yes, something like that would be nice.

> We don’t really want a situation where an unchecked pointer in the
> rare write region completely defeats the mechanism.

Agreed.