netdev - Re: [RFC PATCH 00/17] virtual-bus

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Date:	Thu, 02 Apr 2009 19:19:15 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Anthony Liguori <anthony@...emonkey.ws>
CC:	Gregory Haskins <ghaskins@...ell.com>,
	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	agraf@...e.de, pmullaney@...ell.com, pmorreale@...ell.com,
	rusty@...tcorp.com.au, netdev@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [RFC PATCH 00/17] virtual-bus

Anthony Liguori wrote:
>> I don't think we even need that to end this debate.  I'm convinced we 
>> have a bug somewhere.  Even disabling TX mitigation, I see a ping 
>> latency of around 300ns whereas it's only 50ns on the host.  This 
>> defies logic so I'm now looking to isolate why that is.
>
> I'm down to 90us.  Obviously, s/ns/us/g above.  The exec.c changes 
> were the big winner... I hate qemu sometimes.
>
>

What, this:

> diff --git a/qemu/exec.c b/qemu/exec.c
> index 67f3fa3..1331022 100644
> --- a/qemu/exec.c
> +++ b/qemu/exec.c
> @@ -3268,6 +3268,10 @@ uint32_t ldl_phys(target_phys_addr_t addr)
>      unsigned long pd;
>      PhysPageDesc *p;
>  
> +#if 1
> +    return ldl_p(phys_ram_base + addr);
> +#endif
> +
>      p = phys_page_find(addr >> TARGET_PAGE_BITS);
>      if (!p) {
>          pd = IO_MEM_UNASSIGNED;
> @@ -3300,6 +3304,10 @@ uint64_t ldq_phys(target_phys_addr_t addr)
>      unsigned long pd;
>      PhysPageDesc *p;
>  
> +#if 1
> +    return ldq_p(phys_ram_base + addr);
> +#endif
> +
>      p = phys_page_find(addr >> TARGET_PAGE_BITS);
>      if (!p) {
>          pd = IO_MEM_UNASSIGNED;

The way I read it, it will run only run slowly once per page, then 
settle to a cache miss per page.

Regardless, it makes a memslot model even more attractive.


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

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