Re: [EXTERNAL] [PATCH 2/2] target/ppc: Fix ISA v3.0 (POWER9) slbia implementation

[Nicholas Piggin]

Benjamin Herrenschmidt's on March 19, 2020 6:46 am:
> On Wed, 2020-03-18 at 18:08 +0100, Cédric Le Goater wrote:
>> On 3/18/20 5:41 AM, Nicholas Piggin wrote:
>> > Linux using the hash MMU ("disable_radix" command line) on a POWER9
>> > machine quickly hits translation bugs due to using v3.0 slbia
>> > features that are not implemented in TCG. Add them.
>> 
>> I checked the ISA books and this looks OK but you are also modifying
>> slbie.

That was a mistake that leaked in from debugging the crashes.

> For the same reason, I believe slbie needs to invalidate caches even if
> the entry isn't present.

I don't think it does per the ISA. If we overwrite it then we can only
invalidate with slbia. That's why there is that slb insertion cache for
pre-POWER9 that lets us use slbies to context switch so long as none 
have been overwritten.

> The kernel will under some circumstances overwrite SLB entries without
> invalidating (because the translation itself isn't invalid, it's just
> that the SLB is full, so anything cached in the ERAT is still
> technically ok).
> 
> However, when those things get really invalidated, they need to be
> taken out, even if they no longer have a corresponding SLB entry.

Yeah we track that and do slbia in that case.

Thanks,
Nick