On 2016/8/24 22:36, Benjamin Serebrin wrote:
iommu=off would kill the SWIOTLB as well, while swiotlb=1 consumes 1MB.
However, maintaining guests' kernel commandlines is something we'd
like to stay away from if possible. It's certainly a short-term
I don't quite understand why stay away from kernel command line. It
provides more flexibility, allowing you to turn on/off it by yourself.
answer, or something individual customers can choose to do today.
Thanks!
Ben
On Tue, Aug 23, 2016 at 7:44 PM, Yang Zhang <yang.zhang.wz@xxxxxxxxx> wrote:
On 2016/8/23 7:45, Benjamin Serebrin wrote:
Hi, kvm land,
The Linux SWIOTLB runs by default in our guest images (and likely all
Linux guests in any cloud), even though SWIOTLB will never actually be
used. By default, the SWIOTLB allocates a 64 MB bounce buffer in
contiguous low memory that is never used, so that RAM is wasted. I'd
like to gather opinions on how to tell guests not to bother wasting
that RAM.
Does the kernel parameter swiotlb cannot solve your problem? I usually set
the swiotlb = 1 page to reduce memory wasting for swiotlb.
Possible solutions that we've discussed internally:
- We could have an explicit detection that the guest can use to
decide to not allocate the SWIOTLB.
- The easiest is a hypervisor leaf CPUID bit.
- However, ACPI is arguably a more appropriate place for this kind
of platform information. ACPI is a bit more involved to implement in
guest and BIOS; is it worth the trouble?
- Let the guest infer that no SWIOTLB is needed, perhaps by detecting
the absence of ACPI hotplug slots, and (at late boot) all known PCIe
devices have DMA masks capable of addressing all of RAM.
The timing is nice; we'll see if there's interest in holding a short
BoF at this week's KVM forum.
--
Yang
Alibaba Cloud Computing
--
Yang
Alibaba Cloud Computing
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