Re: [PATCH 07/29] nVMX: Hold a vmcs02 for each vmcs12

[Avi Kivity <avi@redhat.com>]

On 02/03/2011 02:57 PM, Nadav Har'El wrote:
> On Sun, Jan 30, 2011, Avi Kivity wrote about "Re: [PATCH 07/29] nVMX: Hold a vmcs02 for each vmcs12":
> >..
> >  >+static int nested_create_current_vmcs(struct kvm_vcpu *vcpu)
> >  >+{
> >...
> >  >+	if (vmx->nested.vmcs02_num>= NESTED_MAX_VMCS)
> >  >+		return -ENOMEM;
> >
> >  I asked to replace this by dropping the entire vmcs02_list (or perhaps
> >  just its tail).
>
> Hi, here is a completely rewritten patch.
>
> Now we make no guarantee to keep one vmcs02 for each vmcs12. Rather, we
> have a limited pool of vmcs02. If we can find the same vmcs02 that we
> previously used for the current vmcs12. Otherwise, we can take any of them
> (we take the least recently used) and just use that. Of course, if the
> pool is not yet fool, we can also allocate a new vmcs02.
>
> The current default size for the pool is 1, meaning that we just keep one
> vmcs02 (per vcpu) for use in any L2, potentially many of them. Because in
> this version prepare_vmcs02 each time sets all vmcs02 fields, and doesn't
> try to avoid setting rarely modified fields, there's nothing to gain by
> trying to start from the previous vmcs02 used to run a certain L2.
> In the future, when we do have an optimized prepare_vmcs02 which doesn't
> set every field each time, it will make sense to increase the pool size.
>
>
>
> Subject: [PATCH 07/29] nVMX: Introduce vmcs02: VMCS used to run L2
>
> We saw in a previous patch that L1 controls its L2 guest with a vcms12.
> L0 needs to create a real VMCS for running L2. We call that "vmcs02".
> A later patch will contain the code, prepare_vmcs02(), for filling the vmcs02
> fields. This patch only contains code for allocating vmcs02.
>
> In this version, prepare_vmcs02() sets *all* of vmcs02's fields each time we
> enter from L1 to L2, so keeping just one vmcs02 for the vcpu would have
> sufficed: It could be reused even when L1 runs multiple L2 guests.
> However, in future versions we'll probably want to add an optimization where
> vmcs02 fields that rarely change will not be set each time. For that reason
> it is beneficial to keep around several vmcs02s of L2 guests that have
> recently run, so that potentially we could run these L2s again more quickly
> because less vmwrites to vmcs02 will be needed.
>
> This patch adds to each vcpu a vmcs02 pool, vmx->nested.vmcs02_pool,
> which remembers the vmcs02s last used to run up to VMCS02_POOL_SIZE L2s.
> Because in the current version prepare_vmcs02() sets all vmcs02 fields no
> matter what we start with, we choose VMCS02_POOL_SIZE=1. I.e., one vmcs02
> is allocated (and loaded onto the processor), and it is reused to enter any
> L2 guest. In the future, when prepare_vmcs02() is optimized not to set all
> fields every time, VMCS02_POOL_SIZE should be increased.
>

Thanks, that looks much nicer.

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