Re: [PATCH 1/4] kvm: provide synchronous registers in kvm_run

[Avi Kivity <avi@redhat.com>]

On 01/09/2012 03:01 PM, Alexander Graf wrote:
> On 05.01.2012, at 10:54, Christian Borntraeger wrote:
>
> > 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 certain 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.
> > We also provide two 64bit flags fields (architecture specific), that will
> > specify which parts of these fields are valid. Each bit will define that
> > a group of registers (like general purpose) or a single register is valid.
> > In that way we can extend and shrink the interface. (The structure definition
> > itself can only grow of course).
> > 
> > Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
> > ---
> > Documentation/virtual/kvm/api.txt |   28 ++++++++++++++++++++++++++++
> > arch/ia64/include/asm/kvm.h       |    7 +++++++
> > arch/powerpc/include/asm/kvm.h    |    7 +++++++
> > arch/s390/include/asm/kvm.h       |    6 ++++++
> > arch/x86/include/asm/kvm.h        |    7 +++++++
> > include/linux/kvm.h               |   20 ++++++++++++++++++++
> > 6 files changed, 75 insertions(+), 0 deletions(-)
> > 
> > diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
> > index da1f8fd..a149e22 100644
> > --- a/Documentation/virtual/kvm/api.txt
> > +++ b/Documentation/virtual/kvm/api.txt
> > @@ -1693,6 +1693,34 @@ developer registration required to access it).
> > 		/* Fix the size of the union. */
> > 		char padding[256];
> > 	};
> > +
> > +	/*
> > +	 * Here are two fields that allow to access often used registers
> > +         * directly, to avoid the overhead of the ioctl system call. Each
> > +         * register can be extended or reduced by having flag bits specifying
> > +         * if a group of registers is valid.
> > +         */
> > +	__u64 kvm_valid_sync_ro_regs;
> > +	__u64 kvm_valid_sync_rw_regs;
> > +	union {
> > +		/* registers which can be only read */
> > +		struct kvm_sync_ro_regs sync_ro;
> > +		char padding[1024];
> > +	};
> > +	union {
> > +		/* read/write guest registers */
> > +		struct kvm_sync_rw_regs sync_rw;
> > +		char padding[1024];
> > +	};
>
> Ah, new patch set. Same comment here. Please give the union a name. Also, I still don't understand why we need 2 structs. 

Right.

> All we need is a bitmap that says "this field is dirty" and then kernel space can decide to ignore it.

Makes sense.

-- 
error compiling committee.c: too many arguments to function