> On Sep 8, 2017, at 6:05 PM, Linus Torvalds <torva...@linux-foundation.org> > wrote: > >> On Fri, Sep 8, 2017 at 5:00 PM, Andy Lutomirski <l...@kernel.org> wrote: >> >> I'm not convinced. The SDM says (Vol 3, 11.3, under WC): >> >> If the WC buffer is partially filled, the writes may be delayed until >> the next occurrence of a serializing event; such as, an SFENCE or >> MFENCE instruction, CPUID execution, a read or write to uncached >> memory, an interrupt occurrence, or a LOCK instruction execution. >> >> Thanks, Intel, for definiing "serializing event" differently here than >> anywhere else in the whole manual. > > Yeah, it's really badly defined. Ok, maybe a locked instruction does > actually wait for it.. It should be invisible to anything, regardless. > >> 1. The kernel wants to reclaim a page of normal memory, so it unmaps >> it and flushes. Another CPU has an entry for that page in its WC >> buffer. I don't think we care whether the flush causes the WC write >> to really hit RAM because it's unobservable -- we just need to make >> sure it is ordered, as seen by software, before the flush operation >> completes. From the quote above, I think we're okay here. > > Agreed. > >> 2. The kernel is unmapping some IO memory (e.g. a GPU command buffer). >> It wants a guarantee that, when flush_tlb_mm_range returns, all CPUs >> are really done writing to it. Here I'm less convinced. The SDM >> quote certainly suggests to me that we have a promise that the WC >> write has *started* before flush_tlb_mm_range returns, but I'm not >> sure I believe that it's guaranteed to have retired. > > If others have writable TLB entries, what keeps them from just > continuing to write for a long time afterwards?
Whoever unmaps the resource by kicking out their drm fd? I admit I'm just trying to think of the worst case. > >> I'd prefer to leave it as is except on the buggy AMD CPUs, though, >> since the current code is nice and fast. > > So is there a patch to detect the 383 erratum and serialize for those? > I may have missed that part. > The patch is in my head. It's imaginarily attached to this email. > Linus
