On Mon, Mar 9, 2015 at 8:00 AM, Denys Vlasenko <vda.li...@googlemail.com> wrote: > On Mon, Mar 9, 2015 at 3:18 PM, Andy Lutomirski <l...@amacapital.net> wrote: >> Do we actually need espfix on all returns to vm86 mode? > > No, the current code (and my new version) does *not* do > espfix for vm86. It's not needed (apparently). > >>> + btl $X86_EFLAGS_VM_BIT,PT_EFLAGS(%esp) >>> + jc restore_nocheck # VM set, not it >> >> This seems useless. In vm86 mode, espfix should work fine (even if >> pointlessly), CS won't have the two low bits set, and SS won't >> reference the LDT because it's not a selector at all. > > You seem to suggest we can drop VM flag test. > > If we do that, the tests for CS and SS will work on bogus data. > I.e. they will semi-randomly rouse execution through espfix. >
Mmm, right. My bad, that test is needed. > Which will probably work correctly, but IIRC espfix does crazy stuff > which is likely to be slow. > > What we definitely should do here is at least frame this check with > "#ifdef CONFIG_VM86". > >> That being said, what ends up in the high bits of esp when we iret to >> vm86 mode? > > I don't know. I guess it's time to write an actual vm86 testcase :) Ick. I can try... Anyway, you've convinced me that your patch is good. I queued it up. -- Andy Lutomirski AMA Capital Management, LLC -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
