On Wed, Oct 1, 2014 at 7:32 AM, Chuck Ebbert <cebbert.lkml@xxxxxxxxx> wrote: > On Wed, 1 Oct 2014 09:09:13 -0500 > Chuck Ebbert <cebbert.lkml@xxxxxxxxx> wrote: > >> On Tue, 30 Sep 2014 21:51:27 -0700 >> Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote: >> >> > The NT flag doesn't do anything in long mode other than causing IRET >> > to #GP. Oddly, CPL3 code can still set NT using popf. >> > >> > Entry via hardware or software interrupt clears NT automatically, so >> > the only relevant entries are fast syscalls. >> > >> > If user code causes kernel code to run with NT set, then there's at >> > least some (small) chance that it could cause trouble. For example, >> > user code could cause a call to EFI code with NT set, and who knows >> > what would happen? Apparently some games on Wine sometimes do >> > this (!), and, if an IRET return happens, they will segfault. That >> > segfault cannot be handled, because signal delivery fails, too. >> > >> > This patch programs the CPU to clear NT on entry via SYSCALL (both >> > 32-bit and 64-bit, by my reading of the AMD APM), and it clears NT >> > in software on entry via SYSENTER. >> > >> > To save a few cycles, this borrows a trick from Jan Beulich in Xen: >> > it checks whether NT is set before trying to clear it. As a result, >> > it seems to have very little effect on SYSENTER performance on my >> > machine. >> > >> > Testers beware: on Xen, SYSENTER with NT set turns into a GPF. >> > >> > I haven't touched anything on 32-bit kernels. >> > >> > The syscall mask change comes from a variant of this patch by Anish >> > Bhatt. >> > >> > Cc: stable@xxxxxxxxxxxxxxx >> > Reported-by: Anish Bhatt <anish@xxxxxxxxxxx> >> > Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx> >> > --- >> > arch/x86/ia32/ia32entry.S | 12 ++++++++++++ >> > arch/x86/kernel/cpu/common.c | 2 +- >> > 2 files changed, 13 insertions(+), 1 deletion(-) >> > >> > diff --git a/arch/x86/ia32/ia32entry.S b/arch/x86/ia32/ia32entry.S >> > index 4299eb05023c..44d1dd371454 100644 >> > --- a/arch/x86/ia32/ia32entry.S >> > +++ b/arch/x86/ia32/ia32entry.S >> > @@ -151,6 +151,18 @@ ENTRY(ia32_sysenter_target) >> > 1: movl (%rbp),%ebp >> > _ASM_EXTABLE(1b,ia32_badarg) >> > ASM_CLAC >> > + >> > + /* >> > + * Sysenter doesn't filter flags, so we need to clear NT >> > + * ourselves. To save a few cycles, we can check whether >> > + * NT was set instead of doing an unconditional popfq. >> > + */ >> > + testl $X86_EFLAGS_NT,EFLAGS(%rsp) /* saved EFLAGS match cpu */ >> > + jz 1f >> > + pushq_cfi $(X86_EFLAGS_IF|X86_EFLAGS_FIXED) >> > + popfq_cfi >> > +1: >> > + >> >> I think you've gone backwards with this version. The earlier one got >> some of the performance loss back by not needing to do the "cld" insn. >> >> You should just replace that "cld" (line 146) with >> >> pushfq_cfi $2 >> popfq_cfi >> >> Unfortunately I'm not set up to test that yet. But I did look at >> the SDM and can't see a need to preserve any of the flags. >> > > > <sigh> that's: > > pushfw_cfi $0x202 > > IF needs to stay on because we've already enabled interrupts after > sysenter. I tried exactly this. It was much slower than the version I sent. --Andy -- Andy Lutomirski AMA Capital Management, LLC -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html