Re: [PATCH] kvm: Flush coalesced MMIO buffer periodly

[Marcelo Tosatti <mtosatti@redhat.com>]

On Mon, Jan 25, 2010 at 03:46:44PM +0800, Sheng Yang wrote:
> The default action of coalesced MMIO is, cache the writing in buffer, until:
> 1. The buffer is full.
> 2. Or the exit to QEmu due to other reasons.
> 
> But this would result in a very late writing in some condition.
> 1. The each time write to MMIO content is small.
> 2. The writing interval is big.
> 3. No need for input or accessing other devices frequently.
> 
> This issue was observed in a experimental embbed system. The test image
> simply print "test" every 1 seconds. The output in QEmu meets expectation,
> but the output in KVM is delayed for seconds.
> 
> Per Avi's suggestion, I hooked flushing coalesced MMIO buffer in VGA update
> handler. By this way, We don't need vcpu explicit exit to QEmu to
> handle this issue.

Sheng,

Can you send this to QEMU upstream first, since the feature is present
there.

> Signed-off-by: Sheng Yang <sheng@linux.intel.com>
> ---
>  cpu-all.h  |    2 ++
>  exec.c     |    6 ++++++
>  kvm-all.c  |   20 ++++++++++++++++++++
>  qemu-kvm.c |    9 +++++++--
>  qemu-kvm.h |    2 ++
>  vl.c       |    2 ++
>  6 files changed, 39 insertions(+), 2 deletions(-)
> 
> diff --git a/cpu-all.h b/cpu-all.h
> index 8ed76c7..51effc0 100644
> --- a/cpu-all.h
> +++ b/cpu-all.h
> @@ -916,6 +916,8 @@ void qemu_register_coalesced_mmio(target_phys_addr_t addr, ram_addr_t size);
>  
>  void qemu_unregister_coalesced_mmio(target_phys_addr_t addr, ram_addr_t size);
>  
> +void qemu_flush_coalesced_mmio_buffer(void);
> +
>  /*******************************************/
>  /* host CPU ticks (if available) */
>  
> diff --git a/exec.c b/exec.c
> index 99e88e1..40c01a1 100644
> --- a/exec.c
> +++ b/exec.c
> @@ -2424,6 +2424,12 @@ void qemu_unregister_coalesced_mmio(target_phys_addr_t addr, ram_addr_t size)
>          kvm_uncoalesce_mmio_region(addr, size);
>  }
>  
> +void qemu_flush_coalesced_mmio_buffer(void)
> +{
> +    if (kvm_enabled())
> +        kvm_flush_coalesced_mmio_buffer();
> +}
> +
>  #ifdef __linux__
>  
>  #include <sys/vfs.h>
> diff --git a/kvm-all.c b/kvm-all.c
> index 0423fff..3d9fcc0 100644
> --- a/kvm-all.c
> +++ b/kvm-all.c
> @@ -25,6 +25,9 @@
>  #include "hw/hw.h"
>  #include "gdbstub.h"
>  #include "kvm.h"
> +#ifndef KVM_UPSTREAM
> +#include "libkvm.h"
> +#endif
>  
>  #ifdef KVM_UPSTREAM
>  /* KVM uses PAGE_SIZE in it's definition of COALESCED_MMIO_MAX */
> @@ -385,6 +388,23 @@ int kvm_uncoalesce_mmio_region(target_phys_addr_t start, ram_addr_t size)
>      return ret;
>  }
>  
> +void kvm_flush_coalesced_mmio_buffer(void)
> +{
> +#ifdef KVM_CAP_COALESCED_MMIO
> +    if (kvm_state->coalesced_mmio_ring) {
> +        struct kvm_coalesced_mmio_ring *ring =
> +            kvm_state->coalesced_mmio_ring;
> +        while (ring->first != ring->last) {
> +            cpu_physical_memory_rw(ring->coalesced_mmio[ring->first].phys_addr,
> +                           &ring->coalesced_mmio[ring->first].data[0],
> +                           ring->coalesced_mmio[ring->first].len, 1);
> +	    smp_wmb();

Tab breakage.