On Tue, Apr 22, 2014 at 10:29 AM, H. Peter Anvin <h...@zytor.com> wrote: > On 04/22/2014 10:19 AM, Linus Torvalds wrote: >> On Tue, Apr 22, 2014 at 10:11 AM, Andrew Lutomirski <aml...@gmail.com> wrote: >>> >>>> >>>> Anyway, if done correctly, this whole espfix should be totally free >>>> for normal processes, since it should only trigger if SS is a LDT >>>> entry (bit #2 set in the segment descriptor). So the normal fast-path >>>> should just have a simple test for that. >>> >>> How? Doesn't something still need to check whether SS is funny before >>> doing iret? >> >> Just test bit #2. Don't do anything else if it's clear, because you >> should be done. You don't need to do anything special if it's clear, >> because I don't *think* we have any 16-bit data segments in the GDT on >> x86-64. >> > > And we don't (neither do we on i386, and we depend on that invariance.) > > Hence: > > irq_return: > + /* > + * Are we returning to the LDT? Note: in 64-bit mode > + * SS:RSP on the exception stack is always valid. > + */ > + testb $4,(SS-RIP)(%rsp) > + jnz irq_return_ldt > + > +irq_return_iret: > INTERRUPT_RETURN > - _ASM_EXTABLE(irq_return, bad_iret) > + _ASM_EXTABLE(irq_return_iret, bad_iret) > > > That is the whole impact of the IRET path. > > If using IST for #GP won't cause trouble (ISTs don't nest, so we need to > make sure there is absolutely no way we could end up nested) then the > rest of the fixup code can go away and we kill the common path > exception-handling overhead; the only new overhead is the IST > indirection for #GP, which isn't a performance-critical fault (good > thing, because untangling #GP faults is a major effort.)
I'd be a bit nervous about read_msr_safe and friends. Also, what happens if userspace triggers a #GP and the signal stack setup causes a page fault? --Andy -- 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/
