linux-kernel - [PATCH v2] KVM: arm64: ARM_SMCCC_ARCH_WORKAROUND_1 doesn't return SMCCC_RET_NOT

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Date:   Wed, 21 Oct 2020 20:29:58 -0700
From:   Stephen Boyd <swboyd@...omium.org>
To:     Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Andre Przywara <andre.przywara@....com>,
        Steven Price <steven.price@....com>,
        Marc Zyngier <maz@...nel.org>, stable@...r.kernel.org
Subject: [PATCH v2] KVM: arm64: ARM_SMCCC_ARCH_WORKAROUND_1 doesn't return SMCCC_RET_NOT_REQUIRED

According to the SMCCC spec[1](7.5.2 Discovery) the
ARM_SMCCC_ARCH_WORKAROUND_1 function id only returns 0, 1, and
SMCCC_RET_NOT_SUPPORTED.

 0 is "workaround required and safe to call this function"
 1 is "workaround not required but safe to call this function"
 SMCCC_RET_NOT_SUPPORTED is "might be vulnerable or might not be, who knows, I give up!"

SMCCC_RET_NOT_SUPPORTED might as well mean "workaround required, except
calling this function may not work because it isn't implemented in some
cases". Wonderful. We map this SMC call to

 0 is SPECTRE_MITIGATED
 1 is SPECTRE_UNAFFECTED
 SMCCC_RET_NOT_SUPPORTED is SPECTRE_VULNERABLE

For KVM hypercalls (hvc), we've implemented this function id to return
SMCCC_RET_NOT_SUPPORTED, 0, and SMCCC_RET_NOT_REQUIRED. One of those
isn't supposed to be there. Per the code we call
arm64_get_spectre_v2_state() to figure out what to return for this
feature discovery call.

 0 is SPECTRE_MITIGATED
 SMCCC_RET_NOT_REQUIRED is SPECTRE_UNAFFECTED
 SMCCC_RET_NOT_SUPPORTED is SPECTRE_VULNERABLE

Let's clean this up so that KVM tells the guest this mapping:

 0 is SPECTRE_MITIGATED
 1 is SPECTRE_UNAFFECTED
 SMCCC_RET_NOT_SUPPORTED is SPECTRE_VULNERABLE

(Note: Moving SMCCC_RET_NOT_AFFECTED to a header is left out of this
patch as it would need to move from proton-pack.c which isn't in stable
trees and the name isn't actually part of the SMCCC spec)

Cc: Andre Przywara <andre.przywara@....com>
Cc: Steven Price <steven.price@....com>
Cc: Marc Zyngier <maz@...nel.org>
Cc: stable@...r.kernel.org
Link: https://developer.arm.com/documentation/den0028/latest [1]
Fixes: c118bbb52743 ("arm64: KVM: Propagate full Spectre v2 workaround state to KVM guests")
Signed-off-by: Stephen Boyd <swboyd@...omium.org>
---

I see that before commit c118bbb52743 ("arm64: KVM: Propagate full
Spectre v2 workaround state to KVM guests") we had this mapping:

 0 is SPECTRE_MITIGATED
 SMCCC_RET_NOT_SUPPORTED is SPECTRE_VULNERABLE

so the return value '1' wasn't there then. Once the commit was merged we
introduced the notion of NOT_REQUIRED here when it shouldn't have been
introduced.

Changes from v1:
 * Way longer commit text, more background (sorry)
 * Dropped proton-pack part because it was wrong
 * Rebased onto other patch accepted upstream

 arch/arm64/kvm/hypercalls.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/hypercalls.c b/arch/arm64/kvm/hypercalls.c
index 9824025ccc5c..6a62312d7813 100644
--- a/arch/arm64/kvm/hypercalls.c
+++ b/arch/arm64/kvm/hypercalls.c
@@ -31,7 +31,7 @@ int kvm_hvc_call_handler(struct kvm_vcpu *vcpu)
 				val = SMCCC_RET_SUCCESS;
 				break;
 			case SPECTRE_UNAFFECTED:
-				val = SMCCC_RET_NOT_REQUIRED;
+				val = 1;
 				break;
 			}
 			break;

base-commit: 66dd3474702aa98d5844367e1577cdad78ef7c65
-- 
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