On Fri, Jan 29, 2016 at 04:35:35PM -0800, Tony Luck wrote:
On Wed, Jan 13, 2016 at 8:39 PM, Borislav Petkov <bp(a)alien8.de>
> On Wed, Jan 13, 2016 at 03:22:58PM -0800, Tony Luck wrote:
>> Are there some examples of synthetic CPUID bits?
> X86_FEATURE_ALWAYS is one. The others got renamed into X86_BUG_* ones,
> the remaining mechanism is the same, though.
So something like this [gmail will line wrap, but should still be legible]
Then Dan will be able to use:
if (cpu_has(c, X86_FEATURE_MCRECOVERY))
to decide whether to use the (slightly slower, but recovery capable)
or just pick the fastest memcpy() instead.
The most optimal way of alternatively calling two functions would be
something like this, IMO:
alternative_call(memcpy, __mcsafe_copy, X86_FEATURE_MCRECOVERY,
ASM_OUTPUT2("=a" (mcsafe_ret.trapnr), "=d" (mcsafe_ret.remain)),
"D" (dst), "S" (src), "d" (len));
I hope I've not messed up the calling convention but you want the inputs
in %rdi, %rsi, %rdx and the outputs in %rax, %rdx, respectively. Just
check the asm gcc generates and do not trust me :)
The other thing you probably would need to do is create our own
__memcpy() which returns struct mcsafe_ret so that the signatures of
both functions match.
Yeah, it is a bit of jumping through hoops but this way we do a CALL
<func_ptr> directly in asm, without any JMPs or NOPs padding the other
alternatives methods add.
But if you don't care about a small JMP and that is not a hot path, you
could do the simpler:
which adds a JMP or a 5-byte NOP depending on the X86_FEATURE_MCRECOVERY
diff --git a/arch/x86/include/asm/cpufeature.h
index 7ad8c9464297..621e05103633 100644
@@ -106,6 +106,7 @@
#define X86_FEATURE_APERFMPERF ( 3*32+28) /* APERFMPERF */
#define X86_FEATURE_EAGER_FPU ( 3*32+29) /* "eagerfpu" Non lazy FPU restore
#define X86_FEATURE_NONSTOP_TSC_S3 ( 3*32+30) /* TSC doesn't stop in
S3 state */
+#define X86_FEATURE_MCRECOVERY ( 3*32+31) /* cpu has recoverable
Why not write it out?
machine checks */
/* Intel-defined CPU features, CPUID level 0x00000001 (ecx), word 4 */
#define X86_FEATURE_XMM3 ( 4*32+ 0) /* "pni" SSE-3 */
ECO tip #101: Trim your mails when you reply.