Re: [patch 1/3] kvm: provide synchronous registers in kvm_run

[Avi Kivity <avi@redhat.com>]

On 12/22/2011 01:56 PM, Christian Borntraeger wrote:
> From: Christian Borntraeger <borntraeger@de.ibm.com>
>
> On some cpus the overhead for virtualization instructions is in the same
> range as a system call. Having to call multiple ioctls to get set registers
> will make userspace handled exits more expensive than necessary.
> Lets provide two sections in kvm_run to have a shared save area for
> guest registers.
> 1. the first section is read-only, to handle registers that have side-effects
> 2. the second section is read/write, e.g. for general purpose registers.

An alternative is to have a single section, with a bitmask set by
userspace to indicate which registers have not been changed (a la svm's
CleanBits).  Is this worthwhile for s390?

>  	};
> +	/* Here are two fields that allow to access often used registers
> +         * directly, to avoid the overhead of the ioctl system call */
> +	union {
> +		/* registers which can be only read */
> +		struct sync_ro_regs sync_ro_regs;

kvm_

> +		char padding[1024];
> +	};
> +	union {
> +		/* read/write guest registers */
> +		struct sync_rw_regs sync_rw_regs;

kvm_

> +		char padding[1024];
> +	};
>  };

blank line

> +These fields allow userspace to access certain guest registers without
> +having to call SET/GET_*REGS. Thus we can avoid some system call
> +overhead if userspace has to handle the exit. (only available if
> +KVM_CAP_SYNC_REGS is set). The ioctls will still work.
>  
>


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error compiling committee.c: too many arguments to function