linux-kernel - Re: [PATCH 2/5] KVM: SVM: Optimize nested svm msrpm merging

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Date:	Fri, 26 Feb 2010 12:28:24 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Joerg Roedel <joerg.roedel@....com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	Alexander Graf <agraf@...e.de>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [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@....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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