[Qemu-devel] Re: [PATCHv2 10/12] tap: add vhost/vhostfd options

[Paul Brook]

> I'm sympathetic to your arguments though.  As qemu is today, the above
> is definitely the right thing to do.  But ram is always ram and ram
> always has a fixed (albeit non-linear) mapping within a guest.

I think this assumption is unsafe. There are machines where RAM mappings can 
change. It's not uncommon for a chip select (i.e. physical memory address 
region) to be switchable to several different sources, one of which may be 
RAM.  I'm pretty sure this functionality is present (but not actually 
implemented) on some of the current qemu targets.

I agree that changing RAM mappings under an active DMA is a fairly suspect 
thing to do. However I think we need to avoid cache mappings between separate 
DMA transactions i.e. when the guest can know that no DMA will occur, and 
safely remap things.

I'm also of the opinion that virtio devices should behave the same as any 
other device. i.e. if you put a virtio-net-pci device on a PCI bus behind an 
IOMMU, then it should see the same address space as any other PCI device in 
that location.  Apart from anything else, failure to do this breaks nested 
virtualization.  While qemu doesn't currently implement an IOMMU, the DMA 
interfaces have been designed to allow it.

> void cpu_ram_add(target_phys_addr_t start, ram_addr_t size);

We need to support aliased memory regions. For example the ARM RealView boards 
expose the first 256M RAM at both address 0x0 and 0x70000000. It's also common 
for systems to create aliases by ignoring certain address bits. e.g. each sim 
slot is allocated a fixed 256M region. Populating that slot with a 128M stick 
will cause the contents to be aliased in both the top and bottom halves of 
that region.

Paul