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Re: [Qemu-devel] [RFC] Virt machine memory map

From: Alexander Graf
Subject: Re: [Qemu-devel] [RFC] Virt machine memory map
Date: Mon, 20 Jul 2015 13:23:45 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0

On 07/20/15 11:41, Peter Maydell wrote:
On 20 July 2015 at 09:55, Pavel Fedin <address@hidden> wrote:

  In our project we work on a very fast paravirtualized network I/O drivers, 
based  on ivshmem. We
successfully got ivshmem working on ARM, however with one hack.
Currently we have:
--- cut ---
     [VIRT_PCIE_MMIO] =          { 0x10000000, 0x2eff0000 },
     [VIRT_PCIE_PIO] =           { 0x3eff0000, 0x00010000 },
     [VIRT_PCIE_ECAM] =          { 0x3f000000, 0x01000000 },
     [VIRT_MEM] =                { 0x40000000, 30ULL * 1024 * 1024 * 1024 },
--- cut ---
  And MMIO region is not enough for us because we want to have 1GB mapping for 
PCI device. In order
to make it working, we modify the map as follows:
--- cut ---
     [VIRT_PCIE_MMIO] =            { 0x10000000, 0x7eff0000 },
     [VIRT_PCIE_PIO] =           { 0x8eff0000, 0x00010000 },
     [VIRT_PCIE_ECAM] =          { 0x8f000000, 0x01000000 },
     [VIRT_MEM] =             { 0x90000000, 30ULL * 1024 * 1024 * 1024 },
--- cut ---
  The question is - how could we upstream this? I believe modifying 32-bit virt 
memory map this way
is not good. Will it be OK to have different memory map for 64-bit virt ?
I think the theory we discussed at the time of putting in the PCIe
device was that if we wanted this we'd add support for the other
PCIe memory window (which would then live at somewhere above 4GB).
Alex, can you remember what the idea was?

Yes, pretty much. It would give us an upper bound to the amount of RAM that we're able to support, but at least we would be able to support big MMIO regions like for ivshmem.

I'm not really sure where to put it though. Depending on your kernel config Linux supports somewhere between 39 and 48 or so bits of phys address space. And I'd rather not crawl into the PCI hole rat hole that we have on x86 ;).

We could of course also put it just above RAM - but then our device tree becomes really dynamic and heavily dependent on -m.

But to be honest I think we weren't expecting anybody to need
1GB of PCI MMIO space unless it was a video card...

Ivshmem was actually the most likely target that I could've thought of to require big MMIO regions ;).


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