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

[Sean Christopherson <seanjc@google.com>]

On Thu, Aug 25, 2022, Michal Luczaj wrote:
> On 8/25/22 19:03, Sean Christopherson wrote:
> > On Wed, Aug 24, 2022, Michal Luczaj wrote:
> >> 								\
> >> 	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.
> 
> Yeah, it is fragile in that sense. But does it mean code such as
> vmx_tests.c:into_guest_main() or svm_tests.c:svm_of_test_guest() should be moved
> to 32-bit binaries?

Ideally?  Yeah.  In practice, it's just not worth replicating all the nSVM/nVMX
setup for 32-bit binaries.