On 1/12/2018 4:28 AM, David Woodhouse wrote: > On Thu, 2018-01-11 at 17:58 -0600, Tom Lendacky wrote: >> >>> + * These are the bare retpoline primitives for indirect jmp and call. >>> + * Do not use these directly; they only exist to make the ALTERNATIVE >>> + * invocation below less ugly. >>> + */ >>> +.macro RETPOLINE_JMP reg:req >>> + call .Ldo_rop_\@ >>> +.Lspec_trap_\@: >>> + pause > > Note that we never use that one on AMD. You just get 'lfence; jmp *reg' > instead because you promised us that would work.... while Intel said it > would work for a month or two and then said "er, oops, no it doesn't in > all cases." — so we're half-waiting for you lot to do the same thing :)
In theory we never get that one on AMD. But because of the case where we could be running under a hypervisor and might not be able to verify that lfence was made serializing, we would fall back to the generic retpoline. > > You *do* get the RSB-stuffing one though, which is the same. So... Right. > >> Talked with our engineers some more on using pause vs. lfence. Pause is >> not serializing on AMD, so the pause/jmp loop will use power as it is >> speculated over waiting for return to mispredict to the correct target. >> Can this be changed back to lfence? It looked like a very small >> difference in cycles/time. > > That seems reasonable, although at this stage I'm also tempted to > suggest we can do that kind of fine-tuning in a followup patch. Like > the bikeshedding about numbers vs. readable labels. We really need the > IBRS and IBPB patches to be landing on top of this as soon as possible. Yup, I understand. Thanks, Tom > > Paul, the lfence→pause change was only a tiny micro-optimisation on > Intel, wasn't it? Are you happy with changing the implementations of > the RSB stuffing code to use lfence again (or what about 'hlt')? > > It currently looks like this... the capture loop is using 'jmp' to > match the retpoline instead of 'call' as in your examples: > > > #define __FILL_RETURN_BUFFER(reg, nr, sp, uniq) \ > mov $(nr/2), reg; \ > .Ldo_call1_ ## uniq: \ > call .Ldo_call2_ ## uniq; \ > .Ltrap1_ ## uniq: \ > pause; \ > jmp .Ltrap1_ ## uniq; \ > .Ldo_call2_ ## uniq: \ > call .Ldo_loop_ ## uniq; \ > .Ltrap2_ ## uniq: \ > pause; \ > jmp .Ltrap2_ ## uniq; \ > .Ldo_loop_ ## uniq: \ > dec reg; \ > jnz .Ldo_call1_ ## uniq; \ > add $(BITS_PER_LONG/8) * nr, sp; >
