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SubjectRe: [PATCH 2/5] KVM: SVM: Optimize nested svm msrpm merging
On 02/25/2010 07:15 PM, Joerg Roedel wrote:
> This patch optimizes the way the msrpm of the host and the
> guest are merged. The old code merged the 2 msrpm pages
> completly. This code needed to touch 24kb of memory for that
> operation. The optimized variant this patch introduces
> merges only the parts where the host msrpm may contain zero
> bits. This reduces the amount of memory which is touched to
> 48 bytes.
>
> Signed-off-by: Joerg Roedel<joerg.roedel@amd.com>
> ---
> arch/x86/kvm/svm.c | 67 +++++++++++++++++++++++++++++++++++++++++++++-------
> 1 files changed, 58 insertions(+), 9 deletions(-)
>
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index d8d4e35..d15e0ea 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -92,6 +92,9 @@ struct nested_state {
>
> };
>
> +#define MSRPM_OFFSETS 16
> +static u32 msrpm_offsets[MSRPM_OFFSETS] __read_mostly;
> +
> struct vcpu_svm {
> struct kvm_vcpu vcpu;
> struct vmcb *vmcb;
> @@ -436,6 +439,34 @@ err_1:
>
> }
>
> +static void add_msr_offset(u32 offset)
> +{
> + u32 old;
> + int i;
> +
> +again:
> + for (i = 0; i< MSRPM_OFFSETS; ++i) {
> + old = msrpm_offsets[i];
> +
> + if (old == offset)
> + return;
> +
> + if (old != MSR_INVALID)
> + continue;
> +
> + if (cmpxchg(&msrpm_offsets[i], old, offset) != old)
> + goto again;
> +
> + return;
> + }
> +
> + /*
> + * If this BUG triggers the msrpm_offsets table has an overflow. Just
> + * increase MSRPM_OFFSETS in this case.
> + */
> + BUG();
> +}
>

Why all this atomic cleverness? The possible offsets are all determined
statically. Even if you do them dynamically (makes sense when
considering pmu passthrough), it's per-vcpu and therefore single
threaded (just move msrpm_offsets into vcpu context).

> @@ -1846,20 +1882,33 @@ static int nested_svm_vmexit(struct vcpu_svm *svm)
>
> static bool nested_svm_vmrun_msrpm(struct vcpu_svm *svm)
> {
> - u32 *nested_msrpm;
> - struct page *page;
> + /*
> + * This function merges the msr permission bitmaps of kvm and the
> + * nested vmcb. It is omptimized in that it only merges the parts where
> + * the kvm msr permission bitmap may contain zero bits
> + */
>

A comment that describes the entire function can be moved above the
function, freeing a whole tab stop for contents.


--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.



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