Re: [PATCH v4 6/6] KVM: VMX: enable IPI virtualization

[Sean Christopherson]

On Fri, Sep 10, 2021, Sean Christopherson wrote:
> On Mon, Aug 09, 2021, Zeng Guang wrote:
> > +		if (!pages)
> > +			return -ENOMEM;
> > +
> > +		to_kvm_vmx(kvm)->pid_table = (void *)page_address(pages);
> > +		to_kvm_vmx(kvm)->pid_last_index = KVM_MAX_VCPU_ID;
> 
> I don't see the point of pid_last_index if we're hardcoding it to KVM_MAX_VCPU_ID.
> If I understand the ucode pseudocode, there's no performance hit in the happy
> case, i.e. it only guards against out-of-bounds accesses.
> 
> And I wonder if we want to fail the build if this grows beyond an order-1
> allocation, e.g.
> 
> 		BUILD_BUG_ON(PID_TABLE_ORDER > 1);
> 
> Allocating two pages per VM isn't terrible, but 4+ starts to get painful when
> considering the fact that most VMs aren't going to need more than one page.  For
> now I agree the simplicity of not dynamically growing the table is worth burning
> a page.

Ugh, Paolo has queued a series which bumps KVM_MAX_VCPU_ID to 4096[*].  That makes
this an order-3 allocation, which is quite painful.  One thought would be to let
userspace declare the max vCPU it wants to create, not sure if that would work for
xAPIC though.

[*] https://lkml.kernel.org/r/1111efc8-b32f-bd50-2c0f-4c6f506b544b@xxxxxxxxxx