Re: Direct rdtsc call side-effect

Peter Zijlstra
2023-06-01T20:32:03Z

On Thu, Jun 01, 2023 at 07:07:38PM +0000, Steven Noonan wrote:
> One issue is how much overhead it has. This is an instruction that
> normally executes in roughly 50 clock cycles (RDTSC) to 100 clock
> cycles (RDTSCP) on Zen 3. Based on a proof-of-concept I wrote, the
> overhead of trapping and emulating with a signal handler is roughly
> 100x. On my Zen 3 system, it goes up to around 10000 clock cycles per
> trapped read of RDTSCP.

What about kernel based emulation? You could tie it into user_dispatch
and have a user_dispatch tsc offset.

So regular kernel emulation simply returns the native value (keeps the
VDSO working for one), but then from a user_dispatch range, it returns
+offset.

That is; how slow is the below?

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 58b1f208eff5..18175b45db1f 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -645,6 +645,25 @@ static bool fixup_iopl_exception(struct pt_regs *regs)
return true;
}

+static bool fixup_rdtsc_exception(struct pt_regs *regs)
+{
+ unsigned short bytes;
+ u32 eax, edx;
+
+ if (get_user(bytes, (const short __user *)ip))
+ return false;
+
+ if (bytes != 0x0f31)
+ return false;
+
+ asm volatile ("rdtsc", "=a" (eax), "=d" (edx));
+ regs->ax = eax;
+ regs->dx = edx;
+
+ regs->ip += 2;
+ return true;
+}
+
/*
* The unprivileged ENQCMD instruction generates #GPs if the
* IA32_PASID MSR has not been populated. If possible, populate
@@ -752,6 +771,9 @@ DEFINE_IDTENTRY_ERRORCODE(exc_general_protection)
if (fixup_iopl_exception(regs))
goto exit;

+ if (fixup_rdtsc_exception(regs))
+ goto exit;
+
if (fixup_vdso_exception(regs, X86_TRAP_GP, error_code, 0))
goto exit;