linux-kernel - [PATCH AUTOSEL 5.5 031/106] KVM: PPC: Book3S HV: Fix H

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Message-Id: <20200415114226.13103-31-sashal@kernel.org>
Date:   Wed, 15 Apr 2020 07:41:11 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Michael Roth <mdroth@...ux.vnet.ibm.com>, linuxppc-dev@...abs.org,
        David Gibson <david@...son.dropbear.id.au>,
        Paul Mackerras <paulus@...abs.org>,
        Sasha Levin <sashal@...nel.org>, kvm-ppc@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org
Subject: [PATCH AUTOSEL 5.5 031/106] KVM: PPC: Book3S HV: Fix H_CEDE return code for nested guests

From: Michael Roth <mdroth@...ux.vnet.ibm.com>

[ Upstream commit 1f50cc1705350a4697923203fedd7d8fb1087fe2 ]

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@...abs.org
Cc: David Gibson <david@...son.dropbear.id.au>
Cc: Paul Mackerras <paulus@...abs.org>
Signed-off-by: Michael Roth <mdroth@...ux.vnet.ibm.com>
Reviewed-by: David Gibson <david@...son.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@...abs.org>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 arch/powerpc/kvm/book3s_hv.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index ef6aa63b071b3..a1d793b96d2b7 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -3628,6 +3628,7 @@ int kvmhv_p9_guest_entry(struct kvm_vcpu *vcpu, u64 time_limit,
 		if (trap == BOOK3S_INTERRUPT_SYSCALL && !vcpu->arch.nested &&
 		    kvmppc_get_gpr(vcpu, 3) == H_CEDE) {
 			kvmppc_nested_cede(vcpu);
+			kvmppc_set_gpr(vcpu, 3, 0);
 			trap = 0;
 		}
 	} else {
-- 
2.20.1