On 2020-08-13 14:33, Will Deacon wrote:
On Thu, Aug 13, 2020 at 01:48:34PM +0530, Sai Prakash Ranjan wrote:
KRYO4XX gold/big CPU cores are based on Cortex-A76 which has CSV2
bits set and are spectre-v2 safe. But on big.LITTLE systems where
they are coupled with other CPU cores such as the KRYO4XX silver
based on Cortex-A55 which are spectre-v2 safe but do not have CSV2
bits set, the system wide safe value will be set to the lowest value
of CSV2 bits as per FTR_LOWER_SAFE defined for CSV2 bits of register
ID_AA64PFR0_EL1.
This is a problem when booting a guest kernel on gold CPU cores
where it will incorrectly report ARM_SMCCC_ARCH_WORKAROUND_1 warning
and consider them as vulnerable for Spectre variant 2 due to system
wide safe value which is used in kvm emulation code when reading id
registers. One wrong way of fixing this is to set the FTR_HIGHER_SAFE
for CSV2 bits, so instead add the KRYO4XX gold CPU core to the safe
list which will be consulted even when the sanitised read reports
that CSV2 bits are not set for KRYO4XX gold cores.
Reported-by: Stephen Boyd <swboyd@xxxxxxxxxxxx>
Signed-off-by: Sai Prakash Ranjan <saiprakash.ranjan@xxxxxxxxxxxxxx>
---
arch/arm64/kernel/cpu_errata.c | 1 +
1 file changed, 1 insertion(+)
diff --git a/arch/arm64/kernel/cpu_errata.c
b/arch/arm64/kernel/cpu_errata.c
index 6bd1d3ad037a..6cbdd2d98a2a 100644
--- a/arch/arm64/kernel/cpu_errata.c
+++ b/arch/arm64/kernel/cpu_errata.c
@@ -545,6 +545,7 @@ static const struct midr_range
spectre_v2_safe_list[] = {
MIDR_ALL_VERSIONS(MIDR_HISI_TSV110),
MIDR_ALL_VERSIONS(MIDR_QCOM_KRYO_3XX_SILVER),
MIDR_ALL_VERSIONS(MIDR_QCOM_KRYO_4XX_SILVER),
+ MIDR_ALL_VERSIONS(MIDR_QCOM_KRYO_4XX_GOLD),
We shouldn't be putting CPUs in the safe list when they have CSV2
reporting
that they are mitigated in hardware, so I don't think this is the right
approach.
Ok but the only thing I find wrong in this approach is that it is a
redundant
information because CSV2 is already advertising the mitigation, but
again
CSV2 check is done first so it doesn't really hurt to add it to the safe
list because we already know that it is safe.
Sounds more like KVM should advertise CSV2 for the vCPUs if all of the
physical CPUs without CSV2 set are on the safe list. But then again,
KVM
has always been slightly in denial about big.LITTLE because you can't
sensibly expose it to a guest if there are detectable differences...
Sorry but I don't see how the guest kernel will see the CSV2 bits set
for
gold CPU cores without actually adding them to the safe list or reading
the
not sanitised value of ID_AA64PFR0_EL1 ?
Thanks,
Sai
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