[PATCH] KVM: PPC: Book3S HV: Fix H_CEDE return code for nested guests

[Paul Mackerras]

On Tue, Mar 10, 2020 at 04:11:28PM -0500, Michael Roth wrote:
> The h_cede_tm kvm-unit-test currently fails when run inside an L1 guest
> via the guest/nested hypervisor.
> 
>   ./run-tests.sh -v
>   ...
>   TESTNAME=h_cede_tm TIMEOUT=90s ACCEL= ./powerpc/run powerpc/tm.elf -smp 2,threads=2 -machine cap-htm=on -append "h_cede_tm"
>   FAIL h_cede_tm (2 tests, 1 unexpected failures)
> 
> While the test relates to transactional memory instructions, the actual
> failure is due to the return code of the H_CEDE hypercall, which is
> reported as 224 instead of 0. This happens even when no TM instructions
> are issued.
> 
> 224 is the value placed in r3 to execute a hypercall for H_CEDE, and r3
> is where the caller expects the return code to be placed upon return.
> 
> In the case of guest running under a nested hypervisor, issuing H_CEDE
> causes a return from H_ENTER_NESTED. In this case H_CEDE is
> specially-handled immediately rather than later in
> kvmppc_pseries_do_hcall() as with most other hcalls, but we forget to
> set the return code for the caller, hence why kvm-unit-test sees the
> 224 return code and reports an error.
> 
> Guest kernels generally don't check the return value of H_CEDE, so
> that likely explains why this hasn't caused issues outside of
> kvm-unit-tests so far.
> 
> Fix this by setting r3 to 0 after we finish processing the H_CEDE.
> 
> RHBZ: 1778556
> 
> Fixes: 4bad77799fed ("KVM: PPC: Book3S HV: Handle hypercalls correctly when nested")
> Cc: linuxppc-dev at ozlabs.org
> Cc: David Gibson <david at gibson.dropbear.id.au>
> Cc: Paul Mackerras <paulus at ozlabs.org>
> Signed-off-by: Michael Roth <mdroth at linux.vnet.ibm.com>

Thanks, applied to my kvm-ppc-next branch.

Paul.