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From: Anthony Liguori <anthony@codemonkey.ws>
To: Marcelo Tosatti <mtosatti@redhat.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>,
	quintela@redhat.com, qemu-devel@nongnu.org,
	Paul Brook <paul@codesourcery.com>,
	amit.shah@redhat.com, kraxel@redhat.com
Subject: Re: [Qemu-devel] Re: [PATCHv2 10/12] tap: add vhost/vhostfd options
Date: Tue, 02 Mar 2010 12:13:58 -0600	[thread overview]
Message-ID: <4B8D5566.6030308@codemonkey.ws> (raw)
In-Reply-To: <20100302180025.GA27792@amt.cnet>

On 03/02/2010 12:00 PM, Marcelo Tosatti wrote:
>> - it always returns a transient mapping
>> - it may (transparently) bounce
>> - it may fail to bounce, caller must deal
>>
>> The new function I'm proposing has the following semantics:
>>      
> What exactly are the purposes of the new function?
>    

We need an API that can be used to obtain a persistent mapping of a 
given guest physical address.  We don't have that today.  We can 
potentially change cpu_physical_memory_map() to also accommodate this 
use case but that's a second step.

>> - it always returns a persistent mapping
>>      
> For hotplug? What exactly you mean persistent?
>    

Hotplug cannot happen as long as a persistent mapping exists for an 
address within that region.  This is okay.  You cannot have an active 
device driver DMA'ing to a DIMM and then hot unplug it.  The guest is 
responsible for making sure this doesn't happen.

>> - it never bounces
>> - it will only fail if the mapping isn't ram
>>
>> A caller can use the new function to implement an optimization to
>> force the device to only work with real ram.
>>      
> To bypass the address translation in exec.c?
>    

No, the ultimate goal is to convert virtio ring accesses from:

static inline uint32_t vring_desc_len(target_phys_addr_t desc_pa, int i)
{
     target_phys_addr_t pa;
     pa = desc_pa + sizeof(VRingDesc) * i + offsetof(VRingDesc, len);
     return ldl_phys(pa);
}

len = vring_desc_len(vring.desc_pa, i)

To:

len = ldl_w(vring->desc[i].len);

When host == arch, ldl_w is a nop.  Otherwise, it's just a byte swap.  
ldl_phys() today turns into cpu_physical_memory_read() which is very slow.

To support this, we must enforce that when a guest passes us a physical 
address, we can safely obtain a persistent mapping to it.  This is true 
for any ram address.  It's not true for MMIO memory.  We have no way to 
do this with cpu_physical_memory_map().

>>    IOW, this is something we can use in virtio, but very little else.
>> cpu_physical_memory_map can be used in more circumstances.
>>      
> Does not make much sense to me. The qdev<->  memory map mapping seems
> more important. Following your security enhancement drive, you can for
> example check whether the region can actually be mapped by the device
> and deny otherwise, or do whatever host-side memory protection tricks
> you'd like.
>    

It's two independent things.  Part of what makes virtio so complicated 
to convert to proper bus accessors is it's use of 
ldl_phys/stl_phys/etc.  No other device use those functions.  If we 
reduce virtio to just use a map() function, it simplifies the bus 
accessor conversion.

> And "cpu_ram_map" seems like the memory is accessed through cpu context,
> while it is really always device context.
>    

Yes, but that's a separate effort.  In fact, see 
http://wiki.qemu.org/Features/RamAPI vs. 
http://wiki.qemu.org/Features/PCIMemoryAPI

Regards,

Anthony Liguori

  reply	other threads:[~2010-03-02 18:14 UTC|newest]

Thread overview: 70+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2010-02-25 18:27 [Qemu-devel] [PATCHv2 00/12] vhost-net: upstream integration Michael S. Tsirkin
2010-02-25 18:27 ` [Qemu-devel] [PATCHv2 05/12] virtio: add APIs for queue fields Michael S. Tsirkin
2010-02-25 18:49   ` Blue Swirl
2010-02-26 14:53     ` Michael S. Tsirkin
2010-02-25 19:25   ` [Qemu-devel] " Anthony Liguori
2010-02-26  8:46     ` Gleb Natapov
2010-02-25 18:28 ` [Qemu-devel] [PATCHv2 09/12] vhost: vhost net support Michael S. Tsirkin
2010-02-25 19:04   ` [Qemu-devel] " Juan Quintela
2010-02-26 14:32     ` Michael S. Tsirkin
2010-02-26 14:38       ` Anthony Liguori
2010-02-26 14:54         ` Michael S. Tsirkin
2010-02-25 19:44   ` Anthony Liguori
2010-02-26 14:49     ` Michael S. Tsirkin
2010-02-26 15:18       ` Anthony Liguori
2010-02-27 19:38         ` Michael S. Tsirkin
2010-02-28  1:59           ` Paul Brook
2010-02-28 10:15             ` Michael S. Tsirkin
2010-02-28 12:45               ` Paul Brook
2010-02-28 14:44                 ` Michael S. Tsirkin
2010-02-28 15:23                   ` Paul Brook
2010-02-28 15:37                     ` Michael S. Tsirkin
2010-02-28 16:02           ` Anthony Liguori
2010-02-25 18:28 ` [Qemu-devel] [PATCHv2 02/12] kvm: add API to set ioeventfd Michael S. Tsirkin
2010-02-25 19:19   ` [Qemu-devel] " Anthony Liguori
2010-03-02 17:41     ` Michael S. Tsirkin
2010-02-25 18:28 ` [Qemu-devel] [PATCHv2 04/12] virtio: add notifier support Michael S. Tsirkin
2010-02-25 18:28 ` [Qemu-devel] [PATCHv2 01/12] tap: add interface to get device fd Michael S. Tsirkin
2010-02-25 18:28 ` [Qemu-devel] [PATCHv2 07/12] virtio: move typedef to qemu-common Michael S. Tsirkin
2010-02-25 18:28 ` [Qemu-devel] [PATCHv2 10/12] tap: add vhost/vhostfd options Michael S. Tsirkin
2010-02-25 19:47   ` [Qemu-devel] " Anthony Liguori
2010-02-26 14:51     ` Michael S. Tsirkin
2010-02-26 15:23       ` Anthony Liguori
2010-02-27 19:44         ` Michael S. Tsirkin
2010-02-28 16:08           ` Anthony Liguori
2010-02-28 17:19             ` Michael S. Tsirkin
2010-02-28 20:57               ` Anthony Liguori
2010-02-28 21:01                 ` Michael S. Tsirkin
2010-02-28 22:38                   ` Anthony Liguori
2010-02-28 22:39                 ` Paul Brook
2010-03-01 19:27                   ` Michael S. Tsirkin
2010-03-01 21:54                     ` Anthony Liguori
2010-03-02  9:57                       ` Michael S. Tsirkin
2010-03-02 14:07                   ` Anthony Liguori
2010-03-02 14:33                     ` Paul Brook
2010-03-02 14:39                       ` Anthony Liguori
2010-03-02 14:55                         ` Paul Brook
2010-03-02 15:33                           ` Anthony Liguori
2010-03-02 15:53                             ` Paul Brook
2010-03-02 15:56                               ` Michael S. Tsirkin
2010-03-02 16:12                               ` Anthony Liguori
2010-03-02 16:21                                 ` Marcelo Tosatti
2010-03-02 16:12                 ` Marcelo Tosatti
2010-03-02 16:56                   ` Anthony Liguori
2010-03-02 17:00                     ` Michael S. Tsirkin
2010-03-02 18:00                     ` Marcelo Tosatti
2010-03-02 18:13                       ` Anthony Liguori [this message]
2010-03-02 22:41                     ` Paul Brook
2010-03-03 14:15                       ` Anthony Liguori
2010-03-03 14:43                         ` Paul Brook
2010-03-03 16:24                         ` Marcelo Tosatti
2010-02-25 18:28 ` [Qemu-devel] [PATCHv2 11/12] tap: add API to retrieve vhost net header Michael S. Tsirkin
2010-02-25 18:28 ` [Qemu-devel] [PATCHv2 06/12] virtio: add set_status callback Michael S. Tsirkin
2010-02-25 18:28 ` [Qemu-devel] [PATCHv2 08/12] virtio-pci: fill in notifier support Michael S. Tsirkin
2010-02-25 19:30   ` [Qemu-devel] " Anthony Liguori
2010-02-28 20:02     ` Michael S. Tsirkin
2010-02-25 18:28 ` [Qemu-devel] [PATCHv2 03/12] notifier: event notifier implementation Michael S. Tsirkin
2010-02-25 19:22   ` [Qemu-devel] " Anthony Liguori
2010-02-28 19:59     ` Michael S. Tsirkin
2010-02-25 18:28 ` [Qemu-devel] [PATCHv2 12/12] virtio-net: vhost net support Michael S. Tsirkin
2010-02-25 19:49 ` [Qemu-devel] Re: [PATCHv2 00/12] vhost-net: upstream integration Anthony Liguori

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