On Wed, 1 Oct 2014 07:46:54 -0700 Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote: > 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. > Yeah, it looks like a new paravirt op that enables interrupts and clears all the other flags would be the only way to do this without at least some impact on performance. -- 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