[PATCH v6] KVM: SVM: Allow AVIC support on system w/ physical APIC ID > 255

[PATCH v6] KVM: SVM: Allow AVIC support on system w/ physical APIC ID > 255

[Suravee Suthikulpanit]

Expand KVM's mask for the AVIC host physical ID to the full 12 bits defined
by the architecture.  The number of bits consumed by hardware is model
specific, e.g. early CPUs ignored bits 11:8, but there is no way for KVM
to enumerate the "true" size.  So, KVM must allow using all bits, else it
risks rejecting completely legal x2APIC IDs on newer CPUs.

This means KVM relies on hardware to not assign x2APIC IDs that exceed the
"true" width of the field, but presumably hardware is smart enough to tie
the width to the max x2APIC ID.  KVM also relies on hardware to support at
least 8 bits, as the legacy xAPIC ID is writable by software.  But, those
assumptions are unavoidable due to the lack of any way to enumerate the
"true" width.

Cc: stable@vger.kernel.org
Cc: Maxim Levitsky <mlevitsk@redhat.com>
Suggested-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Fixes: 44a95dae1d22 ("KVM: x86: Detect and Initialize AVIC support")
Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
---
 arch/x86/kvm/svm/avic.c | 7 +------
 arch/x86/kvm/svm/svm.h  | 2 +-
 2 files changed, 2 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
index 90364d02f22a..e4cfd8bf4f24 100644
--- a/arch/x86/kvm/svm/avic.c
+++ b/arch/x86/kvm/svm/avic.c
@@ -974,17 +974,12 @@ avic_update_iommu_vcpu_affinity(struct kvm_vcpu *vcpu, int cpu, bool r)
 void avic_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 {
 	u64 entry;
-	/* ID = 0xff (broadcast), ID > 0xff (reserved) */
 	int h_physical_id = kvm_cpu_get_apicid(cpu);
 	struct vcpu_svm *svm = to_svm(vcpu);
 
 	lockdep_assert_preemption_disabled();
 
-	/*
-	 * Since the host physical APIC id is 8 bits,
-	 * we can support host APIC ID upto 255.
-	 */
-	if (WARN_ON(h_physical_id > AVIC_PHYSICAL_ID_ENTRY_HOST_PHYSICAL_ID_MASK))
+	if (WARN_ON(h_physical_id & ~AVIC_PHYSICAL_ID_ENTRY_HOST_PHYSICAL_ID_MASK))
 		return;
 
 	/*
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index 47ef8f4a9358..cede59cd8999 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -565,7 +565,7 @@ extern struct kvm_x86_nested_ops svm_nested_ops;
 #define AVIC_LOGICAL_ID_ENTRY_VALID_BIT			31
 #define AVIC_LOGICAL_ID_ENTRY_VALID_MASK		(1 << 31)
 
-#define AVIC_PHYSICAL_ID_ENTRY_HOST_PHYSICAL_ID_MASK	(0xFFULL)
+#define AVIC_PHYSICAL_ID_ENTRY_HOST_PHYSICAL_ID_MASK	GENMASK_ULL(11, 0)
 #define AVIC_PHYSICAL_ID_ENTRY_BACKING_PAGE_MASK	(0xFFFFFFFFFFULL << 12)
 #define AVIC_PHYSICAL_ID_ENTRY_IS_RUNNING_MASK		(1ULL << 62)
 #define AVIC_PHYSICAL_ID_ENTRY_VALID_MASK		(1ULL << 63)
-- 
2.25.1

Re: [PATCH v6] KVM: SVM: Allow AVIC support on system w/ physical APIC ID > 255

[Suthikulpanit, Suravee]

Paolo,

Do you have any other concerns regarding this patch?

Regards,
Suravee

On 2/11/2022 7:08 AM, Suravee Suthikulpanit wrote:
> Expand KVM's mask for the AVIC host physical ID to the full 12 bits defined
> by the architecture.  The number of bits consumed by hardware is model
> specific, e.g. early CPUs ignored bits 11:8, but there is no way for KVM
> to enumerate the "true" size.  So, KVM must allow using all bits, else it
> risks rejecting completely legal x2APIC IDs on newer CPUs.
> 
> This means KVM relies on hardware to not assign x2APIC IDs that exceed the
> "true" width of the field, but presumably hardware is smart enough to tie
> the width to the max x2APIC ID.  KVM also relies on hardware to support at
> least 8 bits, as the legacy xAPIC ID is writable by software.  But, those
> assumptions are unavoidable due to the lack of any way to enumerate the
> "true" width.
> 
> Cc: stable@vger.kernel.org
> Cc: Maxim Levitsky <mlevitsk@redhat.com>
> Suggested-by: Sean Christopherson <seanjc@google.com>
> Reviewed-by: Sean Christopherson <seanjc@google.com>
> Fixes: 44a95dae1d22 ("KVM: x86: Detect and Initialize AVIC support")
> Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
> ---
>   arch/x86/kvm/svm/avic.c | 7 +------
>   arch/x86/kvm/svm/svm.h  | 2 +-
>   2 files changed, 2 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
> index 90364d02f22a..e4cfd8bf4f24 100644
> --- a/arch/x86/kvm/svm/avic.c
> +++ b/arch/x86/kvm/svm/avic.c
> @@ -974,17 +974,12 @@ avic_update_iommu_vcpu_affinity(struct kvm_vcpu *vcpu, int cpu, bool r)
>   void avic_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>   {
>   	u64 entry;
> -	/* ID = 0xff (broadcast), ID > 0xff (reserved) */
>   	int h_physical_id = kvm_cpu_get_apicid(cpu);
>   	struct vcpu_svm *svm = to_svm(vcpu);
>   
>   	lockdep_assert_preemption_disabled();
>   
> -	/*
> -	 * Since the host physical APIC id is 8 bits,
> -	 * we can support host APIC ID upto 255.
> -	 */
> -	if (WARN_ON(h_physical_id > AVIC_PHYSICAL_ID_ENTRY_HOST_PHYSICAL_ID_MASK))
> +	if (WARN_ON(h_physical_id & ~AVIC_PHYSICAL_ID_ENTRY_HOST_PHYSICAL_ID_MASK))
>   		return;
>   
>   	/*
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index 47ef8f4a9358..cede59cd8999 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -565,7 +565,7 @@ extern struct kvm_x86_nested_ops svm_nested_ops;
>   #define AVIC_LOGICAL_ID_ENTRY_VALID_BIT			31
>   #define AVIC_LOGICAL_ID_ENTRY_VALID_MASK		(1 << 31)
>   
> -#define AVIC_PHYSICAL_ID_ENTRY_HOST_PHYSICAL_ID_MASK	(0xFFULL)
> +#define AVIC_PHYSICAL_ID_ENTRY_HOST_PHYSICAL_ID_MASK	GENMASK_ULL(11, 0)
>   #define AVIC_PHYSICAL_ID_ENTRY_BACKING_PAGE_MASK	(0xFFFFFFFFFFULL << 12)
>   #define AVIC_PHYSICAL_ID_ENTRY_IS_RUNNING_MASK		(1ULL << 62)
>   #define AVIC_PHYSICAL_ID_ENTRY_VALID_MASK		(1ULL << 63)

Re: [PATCH v6] KVM: SVM: Allow AVIC support on system w/ physical APIC ID > 255

[Paolo Bonzini]

On 2/16/22 23:59, Suthikulpanit, Suravee wrote:
> Paolo,
> 
> Do you have any other concerns regarding this patch?

Queued it now, thanks.  I had left it aside for a second because it 
didn't apply cleanly.  I'll include it in 5.18 for now; we'll then have 
to repost this version as a <=5.17 backport for Greg, after the merge 
window.

Paolo

> Regards,
> Suravee
> 
> On 2/11/2022 7:08 AM, Suravee Suthikulpanit wrote:
>> Expand KVM's mask for the AVIC host physical ID to the full 12 bits 
>> defined
>> by the architecture.  The number of bits consumed by hardware is model
>> specific, e.g. early CPUs ignored bits 11:8, but there is no way for KVM
>> to enumerate the "true" size.  So, KVM must allow using all bits, else it
>> risks rejecting completely legal x2APIC IDs on newer CPUs.
>>
>> This means KVM relies on hardware to not assign x2APIC IDs that exceed 
>> the
>> "true" width of the field, but presumably hardware is smart enough to tie
>> the width to the max x2APIC ID.  KVM also relies on hardware to 
>> support at
>> least 8 bits, as the legacy xAPIC ID is writable by software.  But, those
>> assumptions are unavoidable due to the lack of any way to enumerate the
>> "true" width.
>>
>> Cc: stable@vger.kernel.org
>> Cc: Maxim Levitsky <mlevitsk@redhat.com>
>> Suggested-by: Sean Christopherson <seanjc@google.com>
>> Reviewed-by: Sean Christopherson <seanjc@google.com>
>> Fixes: 44a95dae1d22 ("KVM: x86: Detect and Initialize AVIC support")
>> Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
>> ---
>>   arch/x86/kvm/svm/avic.c | 7 +------
>>   arch/x86/kvm/svm/svm.h  | 2 +-
>>   2 files changed, 2 insertions(+), 7 deletions(-)
>>
>> diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
>> index 90364d02f22a..e4cfd8bf4f24 100644
>> --- a/arch/x86/kvm/svm/avic.c
>> +++ b/arch/x86/kvm/svm/avic.c
>> @@ -974,17 +974,12 @@ avic_update_iommu_vcpu_affinity(struct kvm_vcpu 
>> *vcpu, int cpu, bool r)
>>   void avic_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>>   {
>>       u64 entry;
>> -    /* ID = 0xff (broadcast), ID > 0xff (reserved) */
>>       int h_physical_id = kvm_cpu_get_apicid(cpu);
>>       struct vcpu_svm *svm = to_svm(vcpu);
>>       lockdep_assert_preemption_disabled();
>> -    /*
>> -     * Since the host physical APIC id is 8 bits,
>> -     * we can support host APIC ID upto 255.
>> -     */
>> -    if (WARN_ON(h_physical_id > 
>> AVIC_PHYSICAL_ID_ENTRY_HOST_PHYSICAL_ID_MASK))
>> +    if (WARN_ON(h_physical_id & 
>> ~AVIC_PHYSICAL_ID_ENTRY_HOST_PHYSICAL_ID_MASK))
>>           return;
>>       /*
>> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
>> index 47ef8f4a9358..cede59cd8999 100644
>> --- a/arch/x86/kvm/svm/svm.h
>> +++ b/arch/x86/kvm/svm/svm.h
>> @@ -565,7 +565,7 @@ extern struct kvm_x86_nested_ops svm_nested_ops;
>>   #define AVIC_LOGICAL_ID_ENTRY_VALID_BIT            31
>>   #define AVIC_LOGICAL_ID_ENTRY_VALID_MASK        (1 << 31)
>> -#define AVIC_PHYSICAL_ID_ENTRY_HOST_PHYSICAL_ID_MASK    (0xFFULL)
>> +#define AVIC_PHYSICAL_ID_ENTRY_HOST_PHYSICAL_ID_MASK    
>> GENMASK_ULL(11, 0)
>>   #define AVIC_PHYSICAL_ID_ENTRY_BACKING_PAGE_MASK    (0xFFFFFFFFFFULL 
>> << 12)
>>   #define AVIC_PHYSICAL_ID_ENTRY_IS_RUNNING_MASK        (1ULL << 62)
>>   #define AVIC_PHYSICAL_ID_ENTRY_VALID_MASK        (1ULL << 63)

Re: [PATCH v6] KVM: SVM: Allow AVIC support on system w/ physical APIC ID > 255

[Suthikulpanit, Suravee]

On 2/22/2022 1:32 AM, Paolo Bonzini wrote:
> On 2/16/22 23:59, Suthikulpanit, Suravee wrote:
>> Paolo,
>>
>> Do you have any other concerns regarding this patch?
> 
> Queued it now, thanks.  I had left it aside for a second because it didn't apply cleanly.  I'll include it in 5.18 for now; we'll then have to repost this version as a <=5.17 backport for Greg, after the merge window.
> 
> Paolo

Thank you. I'll repost this for <= 5.17 backport after the merge window.

Suravee