On 30/09/2021 11:29, Marc Zyngier wrote:

On Wed, 29 Sep 2021 11:35:46 +0100,
Suzuki K Poulose [off-list ref] wrote:

quoted

On 03/09/2021 10:16, Marc Zyngier wrote:

quoted

Although KVM can be compiled out of the kernel, it cannot be disabled
at runtime. Allow this possibility by introducing a new mode that
will prevent KVM from initialising.

This is useful in the (limited) circumstances where you don't want
KVM to be available (what is wrong with you?), or when you want
to install another hypervisor instead (good luck with that).

Signed-off-by: Marc Zyngier <maz@kernel.org>
---
   Documentation/admin-guide/kernel-parameters.txt |  3 +++
   arch/arm64/include/asm/kvm_host.h               |  1 +
   arch/arm64/kernel/idreg-override.c              |  1 +
   arch/arm64/kvm/arm.c                            | 14 +++++++++++++-
   4 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 91ba391f9b32..cc5f68846434 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt

@@ -2365,6 +2365,9 @@
   	kvm-arm.mode=
   			[KVM,ARM] Select one of KVM/arm64's modes of operation.
   +			none: Forcefully disable KVM and run in nVHE

mode,
+			      preventing KVM from ever initialising.
+
   			nvhe: Standard nVHE-based mode, without support for
   			      protected guests.
   diff --git a/arch/arm64/include/asm/kvm_host.h
b/arch/arm64/include/asm/kvm_host.h
index f8be56d5342b..019490c67976 100644

--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h

@@ -58,6 +58,7 @@
   enum kvm_mode {
   	KVM_MODE_DEFAULT,
   	KVM_MODE_PROTECTED,
+	KVM_MODE_NONE,
   };
   enum kvm_mode kvm_get_mode(void);
   diff --git a/arch/arm64/kernel/idreg-override.c

b/arch/arm64/kernel/idreg-override.c
index d8e606fe3c21..57013c1b6552 100644

--- a/arch/arm64/kernel/idreg-override.c
+++ b/arch/arm64/kernel/idreg-override.c

@@ -95,6 +95,7 @@ static const struct {
   	char	alias[FTR_ALIAS_NAME_LEN];
   	char	feature[FTR_ALIAS_OPTION_LEN];
   } aliases[] __initconst = {
+	{ "kvm-arm.mode=none",		"id_aa64mmfr1.vh=0" },
   	{ "kvm-arm.mode=nvhe",		"id_aa64mmfr1.vh=0" },
   	{ "kvm-arm.mode=protected",	"id_aa64mmfr1.vh=0" },
   	{ "arm64.nobti",		"id_aa64pfr1.bt=0" },

diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index fe102cd2e518..cdc70e238316 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c

@@ -2064,6 +2064,11 @@ int kvm_arch_init(void *opaque)
   		return -ENODEV;
   	}
   +	if (kvm_get_mode() == KVM_MODE_NONE) {
+		kvm_info("KVM disabled from command line\n");
+		return -ENODEV;
+	}
+
   	in_hyp_mode = is_kernel_in_hyp_mode();
     	if (cpus_have_final_cap(ARM64_WORKAROUND_DEVICE_LOAD_ACQUIRE)

||

@@ -2137,8 +2142,15 @@ static int __init early_kvm_mode_cfg(char *arg)
   		return 0;
   	}
   -	if (strcmp(arg, "nvhe") == 0 &&

!WARN_ON(is_kernel_in_hyp_mode()))
+	if (strcmp(arg, "nvhe") == 0 && !WARN_ON(is_kernel_in_hyp_mode())) {
+		kvm_mode = KVM_MODE_DEFAULT;
   		return 0;
+	}
+
+	if (strcmp(arg, "none") == 0 && !WARN_ON(is_kernel_in_hyp_mode())) {

nit: Does this really need to WARN here ? Unlike the "nvhe" case, if the
user wants to keep the KVM out of the picture for, say debugging
something, it is perfectly Ok to allow the kernel to be running at EL2
without having to change the Firmware to alter the landing EL for the
kernel ?

Well, the doc says "run in nVHE mode" and the option forces
id_aa64mmfr1.vh=0. The WARN_ON() will only fires on broken^Wfruity HW
that is VHE only. Note that this doesn't rely on any firmware change
(we drop from EL2 to EL1 and stay there).

Ah, ok. So the "none" is in fact "nvhe + no-kvm". Thats the bit I
missed. TBH, that name to me sounds like "no KVM" at all, which is what
we want. The question is, do we really need "none" to force vh == 0 ? I
understand this is only a problem on a rare set of HWs. But the generic
option looks deceiving.

That said, I am happy to leave this as is and the doc says so.

We could add another option (none-vhe?) that stays at EL2 and still
disables KVM if there is an appetite for it.

Na. Don't think that is necessary.

quoted

Otherwise,

Acked-by: Suzuki K Poulose <suzuki.poulose@arm.com>

Suzuki

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