On 10.10.19 10:20, Thomas Huth wrote:
> On 10/10/2019 09.52, Christian Borntraeger wrote:
>> similar to commit 2c57da356800 ("selftests: kvm: fix sync_regs_test with
>> newer gccs") and commit 204c91eff798a ("KVM: selftests: do not blindly
>> clobber registers in guest asm") we better do not rely on gcc leaving
>> r11 untouched. We can write the simple ucall inline and have the guest
>> code completely as small assembler function.
>>
>> Signed-off-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
>> Suggested-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
>> ---
>> .../testing/selftests/kvm/s390x/sync_regs_test.c | 15 +++++++++------
>> 1 file changed, 9 insertions(+), 6 deletions(-)
>>
>> diff --git a/tools/testing/selftests/kvm/s390x/sync_regs_test.c b/tools/testing/selftests/kvm/s390x/sync_regs_test.c
>> index d5290b4ad636..04d6abe68961 100644
>> --- a/tools/testing/selftests/kvm/s390x/sync_regs_test.c
>> +++ b/tools/testing/selftests/kvm/s390x/sync_regs_test.c
>> @@ -25,12 +25,15 @@
>>
>> static void guest_code(void)
>> {
>> - register u64 stage asm("11") = 0;
>> -
>> - for (;;) {
>> - GUEST_SYNC(0);
>> - asm volatile ("ahi %0,1" : : "r"(stage));
>> - }
>> + /*
>> + * we embed diag 501 here instead of a ucall as the called function
>> + * could mess with the content of r11 when doing the hypercall
>
> As far as I understood the issue on x86, it's not the called function
> that is messing with the counter register (since r11 is not volatile
> between function calls), but the compiler shuffled it around here
> between the GUEST_SYNC and the inline assembler code. So I'd prefer a
> comment like in the x86 version:
It is actually both.
The called function is GUEST_SYNC (which is ucall).
r11 is call-saved, so when we return from GUEST_SYNC r11 is as it is supposed
to be. The problem is that this function (ucall) is allowed to use r11 for anything
as long as it restores it before returning. As we do the guest exit in ucall
then the test code could see some random value for r11.
>
> /*
> * We embed diag 501 here instead of doing a ucall to avoid that
> * the compiler reshuffles registers before calling the function.
> */
>
> ?
What about
> /*
> * We embed diag 501 here instead of doing a ucall to avoid that
> * the compiler has messed with r11 at the time of the ucall.
> */
>
> With such an updated comment:
>
> Reviewed-by: Thomas Huth <thuth@xxxxxxxxxx>
> > + */
>> + asm volatile (
>> + "0: diag 0,0,0x501\n"
>> + " ahi 11,1\n"
>> + " j 0b\n"
>> + );
>> }
>
