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From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: Eduardo Habkost <ehabkost@redhat.com>, Juergen Gross <jgross@suse.com>
Cc: kvm@vger.kernel.org, x86@kernel.org,
	linux-kernel@vger.kernel.org, maz@kernel.org,
	Paolo Bonzini <pbonzini@redhat.com>,
	Sean Christopherson <seanjc@google.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	"H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH v2 3/6] x86/kvm: introduce per cpu vcpu masks
Date: Wed, 08 Sep 2021 10:41:51 +0200	[thread overview]
Message-ID: <87r1dz4fxs.fsf@vitty.brq.redhat.com> (raw)
In-Reply-To: <20210907183457.53ws6tqqqpzkeil4@habkost.net>

Eduardo Habkost <ehabkost@redhat.com> writes:

> On Fri, Sep 03, 2021 at 03:08:04PM +0200, Juergen Gross wrote:
>> In order to support high vcpu numbers per guest don't use on stack
>> vcpu bitmasks. As all those currently used bitmasks are not used in
>> functions subject to recursion it is fairly easy to replace them with
>> percpu bitmasks.
>> 
>> Disable preemption while such a bitmask is being used in order to
>> avoid double usage in case we'd switch cpus.
>> 
>> Signed-off-by: Juergen Gross <jgross@suse.com>
>> ---
>> V2:
>> - use local_lock() instead of preempt_disable() (Paolo Bonzini)
>> ---
>>  arch/x86/include/asm/kvm_host.h | 10 ++++++++++
>>  arch/x86/kvm/hyperv.c           | 25 ++++++++++++++++++-------
>>  arch/x86/kvm/irq_comm.c         |  9 +++++++--
>>  arch/x86/kvm/x86.c              | 22 +++++++++++++++++++++-
>>  4 files changed, 56 insertions(+), 10 deletions(-)
>> 
>> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
>> index 3513edee8e22..a809a9e4fa5c 100644
>> --- a/arch/x86/include/asm/kvm_host.h
>> +++ b/arch/x86/include/asm/kvm_host.h
>> @@ -15,6 +15,7 @@
>>  #include <linux/cpumask.h>
>>  #include <linux/irq_work.h>
>>  #include <linux/irq.h>
>> +#include <linux/local_lock.h>
>>  
>>  #include <linux/kvm.h>
>>  #include <linux/kvm_para.h>
>> @@ -1591,6 +1592,15 @@ extern bool kvm_has_bus_lock_exit;
>>  /* maximum vcpu-id */
>>  unsigned int kvm_max_vcpu_id(void);
>>  
>> +/* per cpu vcpu bitmasks, protected by kvm_pcpu_mask_lock */
>> +DECLARE_PER_CPU(local_lock_t, kvm_pcpu_mask_lock);
>> +extern unsigned long __percpu *kvm_pcpu_vcpu_mask;
>> +#define KVM_VCPU_MASK_SZ	\
>> +	(sizeof(*kvm_pcpu_vcpu_mask) * BITS_TO_LONGS(KVM_MAX_VCPUS))
>> +extern u64 __percpu *kvm_hv_vp_bitmap;
>> +#define KVM_HV_MAX_SPARSE_VCPU_SET_BITS DIV_ROUND_UP(KVM_MAX_VCPUS, 64)
>> +#define KVM_HV_VPMAP_SZ		(sizeof(u64) * KVM_HV_MAX_SPARSE_VCPU_SET_BITS)
>
> I have just realized that the Hyper-V sparse bitmap format can
> support only up to 4096 CPUs, and the current implementation of
> sparse_set_to_vcpu_mask() won't even work correctly if
> KVM_MAX_VCPUS is larger than 4096.

Nice catch! Indeed, we can only encode 64 'banks' with 64 vCPUs each. We
need to enforce the limit somehow. As a big hammer, I can suggest to
fail kvm_hv_vcpu_init() and write to HV_X64_MSR_VP_INDEX for vCPUs above
4095 for now (I seriously doubt there's real need to run such big
Windows guests anywhere but who knows).

>
> This means vp_bitmap can't and will never be larger than 512
> bytes.  Isn't a per-CPU variable for vp_bitmap overkill in this
> case?

Yes, it's OK to allocate 512 bytes on stack.

-- 
Vitaly


  reply	other threads:[~2021-09-08  8:42 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-03 13:08 [PATCH v2 0/6] x86/kvm: add boot parameters for max vcpu configs Juergen Gross
2021-09-03 13:08 ` [PATCH v2 1/6] x86/kvm: remove non-x86 stuff from arch/x86/kvm/ioapic.h Juergen Gross
2021-09-03 13:08 ` [PATCH v2 2/6] x86/kvm: add boot parameter for adding vcpu-id bits Juergen Gross
2021-09-03 13:43   ` Vitaly Kuznetsov
2021-09-03 13:53     ` Juergen Gross
2021-09-03 19:48   ` Eduardo Habkost
2021-09-06  4:46     ` Juergen Gross
2021-09-28 16:41   ` Paolo Bonzini
2021-09-03 13:08 ` [PATCH v2 3/6] x86/kvm: introduce per cpu vcpu masks Juergen Gross
2021-09-03 16:05   ` Eduardo Habkost
2021-09-06  4:34     ` Juergen Gross
2021-09-07 18:34   ` Eduardo Habkost
2021-09-08  8:41     ` Vitaly Kuznetsov [this message]
2021-09-03 13:08 ` [PATCH v2 4/6] kvm: use kvfree() in kvm_arch_free_vm() Juergen Gross
2021-09-28 16:48   ` Paolo Bonzini
2021-09-03 13:08 ` [PATCH v2 5/6] kvm: allocate vcpu pointer array separately Juergen Gross
2021-09-03 14:41   ` Marc Zyngier
2021-09-06  4:33     ` Juergen Gross
2021-09-06  9:46       ` Marc Zyngier
2021-09-09 20:28         ` Sean Christopherson
2021-09-03 13:08 ` [PATCH v2 6/6] x86/kvm: add boot parameter for setting max number of vcpus per guest Juergen Gross
2021-09-06  0:45   ` Yao Yuan
2021-09-06  4:47     ` Juergen Gross

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