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

[Sean Christopherson <seanjc@google.com>]

On Sat, Sep 18, 2021, Zeng Guang wrote:
> On 9/11/2021 7:55 AM, Sean Christopherson wrote:
> > 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@redhat.com
> Thus we keep current design as no change.

Not necessarily.  I was pointing out that the current design is already problematic
from a memory allocation perspective.  Burning a few pages per vCPU isn't the end
of the world, but 32kb of _contiguous_ memory is rough, especially when 28kb is
unlikely to be used in many cases.