linux-kernel - Re: [PATCH v3 3/4] x86/kvm: add max number of vcpus for hyperv emulation

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Message-ID: <bfe38122-0ddd-d9bc-4927-942b051a39c4@suse.com>
Date:   Thu, 18 Nov 2021 08:43:31 +0100
From:   Juergen Gross <jgross@...e.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     kvm@...r.kernel.org, x86@...nel.org, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v3 3/4] x86/kvm: add max number of vcpus for hyperv
 emulation

On 17.11.21 21:50, Sean Christopherson wrote:
> On Tue, Nov 16, 2021, Juergen Gross wrote:
>> When emulating Hyperv the theoretical maximum of vcpus supported is
>> 4096, as this is the architectural limit for sending IPIs via the PV
>> interface.
>>
>> For restricting the actual supported number of vcpus for that case
>> introduce another define KVM_MAX_HYPERV_VCPUS and set it to 1024, like
>> today's KVM_MAX_VCPUS. Make both values unsigned ones as this will be
>> needed later.
>>
>> The actual number of supported vcpus for Hyperv emulation will be the
>> lower value of both defines.
>>
>> This is a preparation for a future boot parameter support of the max
>> number of vcpus for a KVM guest.
>>
>> Signed-off-by: Juergen Gross <jgross@...e.com>
>> ---
>> V3:
>> - new patch
>> ---
>>   arch/x86/include/asm/kvm_host.h |  3 ++-
>>   arch/x86/kvm/hyperv.c           | 15 ++++++++-------
>>   2 files changed, 10 insertions(+), 8 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
>> index 886930ec8264..8ea03ff01c45 100644
>> --- a/arch/x86/include/asm/kvm_host.h
>> +++ b/arch/x86/include/asm/kvm_host.h
>> @@ -38,7 +38,8 @@
>>   
>>   #define __KVM_HAVE_ARCH_VCPU_DEBUGFS
>>   
>> -#define KVM_MAX_VCPUS 1024
>> +#define KVM_MAX_VCPUS 1024U
>> +#define KVM_MAX_HYPERV_VCPUS 1024U
> 
> I don't see any reason to put this in kvm_host.h, it should never be used outside
> of hyperv.c.

Okay, fine with me.

> 
>>   #define KVM_MAX_VCPU_IDS kvm_max_vcpu_ids()
>>   /* memory slots that are not exposed to userspace */
>>   #define KVM_PRIVATE_MEM_SLOTS 3
>> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
>> index 4a555f32885a..c0fa837121f1 100644
>> --- a/arch/x86/kvm/hyperv.c
>> +++ b/arch/x86/kvm/hyperv.c
>> @@ -41,7 +41,7 @@
>>   /* "Hv#1" signature */
>>   #define HYPERV_CPUID_SIGNATURE_EAX 0x31237648
>>   
>> -#define KVM_HV_MAX_SPARSE_VCPU_SET_BITS DIV_ROUND_UP(KVM_MAX_VCPUS, 64)
>> +#define KVM_HV_MAX_SPARSE_VCPU_SET_BITS DIV_ROUND_UP(KVM_MAX_HYPERV_VCPUS, 64)
>>   
>>   static void stimer_mark_pending(struct kvm_vcpu_hv_stimer *stimer,
>>   				bool vcpu_kick);
>> @@ -166,7 +166,7 @@ static struct kvm_vcpu *get_vcpu_by_vpidx(struct kvm *kvm, u32 vpidx)
>>   	struct kvm_vcpu *vcpu = NULL;
>>   	int i;
>>   
>> -	if (vpidx >= KVM_MAX_VCPUS)
>> +	if (vpidx >= min(KVM_MAX_VCPUS, KVM_MAX_HYPERV_VCPUS))
> 
> IMO, this is conceptually wrong.  KVM should refuse to allow Hyper-V to be enabled
> if the max number of vCPUs exceeds what can be supported, or should refuse to create

TBH, I wasn't sure where to put this test. Is there a guaranteed
sequence of ioctl()s regarding vcpu creation (or setting the max
number of vcpus) and the Hyper-V enabling?

> the vCPUs.  I agree it makes sense to add a Hyper-V specific limit, since there are
> Hyper-V structures that have a hard limit, but detection of violations should be a
> BUILD_BUG_ON, not a silent failure at runtime.
> 

A BUILD_BUG_ON won't be possible with KVM_MAX_VCPUS being selecteble via
boot parameter.


Juergen

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