Re: [PATCH kvm-unit-tests v2] vmexit: time the number of cycles for simple PIO

[Gleb Natapov <gleb@redhat.com>]

On Thu, Dec 13, 2012 at 01:40:37PM +0100, Paolo Bonzini wrote:
> This patch adds three scenarios to the vmexit test.  Two are very simple
> PIO cases that are handled in the kernel (reading from and writing
> to ELCR).  The other is an unmapped PIO that is handled in userspace.
> 
> The difference between the two reading scenarios is roughly the cost of a
> userspace exit; the existing inl_from_pmtimer test is not precise enough,
> because the device model has a pretty high cost.
> 
> The difference between the kernel read and write is the cost of emulation,
> because inl_from_kernel goes through the whole emulation stuff while outl
> does not (it is used for virtio, while the speed of inl matters less).
> 
> Example:
> 
>     vmcall 3898
>     inl_from_pmtimer 24615
>     inl_from_qemu 20574
>     inl_from_kernel 7237
>     outl_to_kernel 4451
> 
> So the cost of exiting to userspace is 13000 cycles on this machine,
> and the cost of emulation is 3300 cycles.
> 
> Suggested-by: Avi Kivity <avi.kivity@gmail.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Thanks, applied. On my machine out is faster than vmcall.

vmcall 2681
outl_to_kernel 2358

> ---
>  x86/vmexit.c | 30 ++++++++++++++++++++++++++++++
>  1 file changed, 30 insertions(+)
> 
> diff --git a/x86/vmexit.c b/x86/vmexit.c
> index ad8ab55..98f0ead 100644
> --- a/x86/vmexit.c
> +++ b/x86/vmexit.c
> @@ -4,6 +4,18 @@
>  #include "processor.h"
>  #include "atomic.h"
>  
> +static void outb(unsigned short port, int val)
> +{
> +    asm volatile("outb %b0, %w1" : "=a"(val) : "Nd"(port));
> +}
> +
> +static unsigned int inb(unsigned short port)
> +{
> +    unsigned int val;
> +    asm volatile("xorl %0, %0; inb %w1, %b0" : "=a"(val) : "Nd"(port));
> +    return val;
> +}
> +
>  static unsigned int inl(unsigned short port)
>  {
>      unsigned int val;
> @@ -82,6 +94,21 @@ static void inl_pmtimer(void)
>      inl(0xb008);
>  }
>  
> +static void inl_nop_qemu(void)
> +{
> +    inl(0x1234);
> +}
> +
> +static void inl_nop_kernel(void)
> +{
> +    inb(0x4d0);
> +}
> +
> +static void outl_elcr_kernel(void)
> +{
> +    outb(0x4d0, 0);
> +}
> +
>  static void ple_round_robin(void)
>  {
>  	struct counter {
> @@ -116,6 +143,9 @@ static struct test {
>  	{ mov_to_cr8, "mov_to_cr8" , .parallel = 1, },
>  #endif
>  	{ inl_pmtimer, "inl_from_pmtimer", .parallel = 1, },
> +	{ inl_nop_qemu, "inl_from_qemu", .parallel = 1 },
> +	{ inl_nop_kernel, "inl_from_kernel", .parallel = 1 },
> +	{ outl_elcr_kernel, "outl_to_kernel", .parallel = 1 },
>  	{ ipi, "ipi", is_smp, .parallel = 0, },
>  	{ ipi_halt, "ipi+halt", is_smp, .parallel = 0, },
>  	{ ple_round_robin, "ple-round-robin", .parallel = 1 },
> -- 
> 1.8.0.2

--
			Gleb.