[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [PATCH] x86/boot: Fix early exception handling with CONFIG_PERF_COUNTERS

Hi Andrew,

On 15/04/2020 18:39, Andrew Cooper wrote:
The PERFC_INCR() macro uses current->processor, but current is not valid
during early boot.  This causes the following crash to occur if
e.g. rdmsr_safe() has to recover from a #GP fault.

   (XEN) Early fatal page fault at e008:ffff82d0803b1a39 (cr2=0000000000000004, 
   (XEN) ----[ Xen-4.14-unstable  x86_64  debug=y   Not tainted ]----
   (XEN) CPU:    0
   (XEN) RIP:    e008:[<ffff82d0803b1a39>] 
   (XEN) Xen call trace:
   (XEN)    [<ffff82d0803b1a39>] R 
   (XEN)    [<ffff82d0806394fe>] F __start_xen+0x2cd/0x2980
   (XEN)    [<ffff82d0802000ec>] F __high_start+0x4c/0x4e

Furthermore, the PERFC_INCR() macro is wildly inefficient.  There has been a
single caller for many releases now, so inline it and delete the macro

For the assembly, move entry_vector from %eax into %ecx.  There is no encoding
benefit for movzbl, and it frees up %eax to be used inside the
CONFIG_PERF_COUNTERS block where there is an encoding benefit.

There is no need to reference current at all.  What is actually needed is the
per_cpu_offset which can be obtained directly from the top-of-stack block.
This simplifies the counter handling to 3 instructions and no spilling to the
stack at all.

The same breakage from above is now handled properly:

   (XEN) traps.c:1591: GPF (0000): ffff82d0806394fe [__start_xen+0x2cd/0x2980] 
-> ffff82d0803b3bfb

Reported-by: Julien Grall <jgrall@xxxxxxxxxx>
Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

I can confirm the crash I have seen has now disappeared.

Tested-by: Julien Grall <jgrall@xxxxxxxxxx>


Julien Grall



Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.