> On Mar 4, 2020, at 10:19 AM, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote: > > On 04/03/20 16:32, Peter Xu wrote: >>> Looks good, thanks. But it seems we should also take care of the comment in __do_insn_fetch_bytes(), as we do not >>> load instruction at the beginning of x86_decode_insn() now, which may be misleading: >>> /* >>> * One instruction can only straddle two pages, >>> * and one has been loaded at the beginning of >>> * x86_decode_insn. So, if not enough bytes >>> * still, we must have hit the 15-byte boundary. >>> */ >>> if (unlikely(size < op_size)) >>> return emulate_gp(ctxt, 0); >> Right, thanks for spotting that (even if the patch to be dropped :). >> >> I guess not only the comment, but the check might even fail if we >> apply the patch. Because when the fetch is the 1st attempt and >> unluckily that acrosses one page boundary (because we'll only fetch >> until either 15 bytes or the page boundary), so that single fetch >> could be smaller than op_size provided. > > Right, priming the decode cache with one byte from the current page > cannot fail, and then we know that the next call must be at the > beginning of the next page. IIRC I encountered (and fixed) a similar KVM bug in the past. It is a shame I never wrote a unit test (and I don’t have time now), but it would be nice to have one.
