Re: [PATCH] KVM: selftests: fix sync_with_host() in smm_test

From: Paolo Bonzini
Date: Wed Jun 10 2020 - 19:54:31 EST


On 10/06/20 18:41, Vitaly Kuznetsov wrote:
> It was reported that older GCCs compile smm_test in a way that breaks
> it completely:
>
> kvm_exit: reason EXIT_CPUID rip 0x4014db info 0 0
> func 7ffffffd idx 830 rax 0 rbx 0 rcx 0 rdx 0, cpuid entry not found
> ...
> kvm_exit: reason EXIT_MSR rip 0x40abd9 info 0 0
> kvm_msr: msr_read 487 = 0x0 (#GP)
> ...
>
> Note, '7ffffffd' was supposed to be '80000001' as we're checking for
> SVM. Dropping '-O2' from compiler flags help. Turns out, asm block in
> sync_with_host() is wrong. We us 'in 0xe, %%al' instruction to sync
> with the host and in 'AL' register we actually pass the parameter
> (stage) but after sync 'AL' gets written to but GCC thinks the value
> is still there and uses it to compute 'EAX' for 'cpuid'.
>
> smm_test can't fully use standard ucall() framework as we need to
> write a very simple SMI handler there. Fix the immediate issue by
> making RAX input/output operand. While on it, make sync_with_host()
> static inline.
>
> Reported-by: Marcelo Bandeira Condotta <mcondotta@xxxxxxxxxx>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> ---
> tools/testing/selftests/kvm/x86_64/smm_test.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/tools/testing/selftests/kvm/x86_64/smm_test.c b/tools/testing/selftests/kvm/x86_64/smm_test.c
> index 36314152943d..ae39a220609f 100644
> --- a/tools/testing/selftests/kvm/x86_64/smm_test.c
> +++ b/tools/testing/selftests/kvm/x86_64/smm_test.c
> @@ -47,10 +47,10 @@ uint8_t smi_handler[] = {
> 0x0f, 0xaa, /* rsm */
> };
>
> -void sync_with_host(uint64_t phase)
> +static inline void sync_with_host(uint64_t phase)
> {
> asm volatile("in $" XSTR(SYNC_PORT)", %%al \n"
> - : : "a" (phase));
> + : "+a" (phase));
> }
>
> void self_smi(void)
>

Queued, thanks.

Paolo