Re: [kvm-unit-tests PATCH] x86/emulator: Test POP-SS blocking

[Sean Christopherson <seanjc@google.com>]

On Wed, Aug 24, 2022, Michal Luczaj wrote:
> On 8/24/22 20:32, Sean Christopherson wrote:
> > Eh, let's completely skip usermode for code #DBs and not tweak __run_single_step_db_test().
> > It's easier to just have a standalone function.
> 
> Something like this?
> 
> static void test_pop_ss_code_db(bool fep_available)
> {
> 	write_ss(KERNEL_DS);
> 
> 	write_dr7(DR7_FIXED_1 |
> 		  DR7_ENABLE_DRx(0) |
> 		  DR7_EXECUTE_DRx(0) |
> 		  DR7_LEN_1_DRx(0));
> 
> #define POPSS_DB(desc, fep1, fep2)				\
> ({								\
> 	unsigned int r;						\
> 								\
> 	n = 0;							\
> 	asm volatile(/* jump to 32-bit code segment */		\
> 		     "ljmp *1f\n\t"				\
> 		     "1:\n\t"					\
> 		     "	.long 2f\n\t"				\
> 		     "	.word " xstr(KERNEL_CS32) "\n\t"	\
> 		     /* exercise POP SS blocking */		\
> 		     ".code32\n\t"				\
> 		     "2: lea 3f, %0\n\t"			\
> 		     "mov %0, %%dr0\n\t"			\
> 		     "push %%ss\n\t"				\
> 		     fep1 "pop %%ss\n\t"			\
> 		     fep2 "3: xor %0, %0\n\t"			\
> 		     /* back to long mode */			\
> 		     "ljmp %[cs64], $4f\n\t"			\
> 		     ".code64\n\t"				\

Ooh, I see what you meant by temporarily switching to 32-bit mode.  I was thinking
we could just make the POP SS testcase 32-bit only, but I didn't realize this test
is 64-bit only.  Argh, and so is emulate.c.  And now I get why you added a brand
new test.

Let's just add a new test.  The above can work, but it relies on the code and
stack being mapped with a 32-bit address, e.g. will break if KUT is ever changed
to not map everything low in the virtual address space.

I think it makes sense to rename emulator.c => emulator64.c, and then start a new
emulator.c for tests that apply to both 32-bit and 64-bit KUT.

I'll send a small series, the behavior is also different for AMD CPUs (I coded up
99% of this yesterday before realizing this morning that debug.c is 64-bit only).