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Christophe, looks like the bug you mentioned is not public yet, alteast I am not able to access it.
Agree with disabling DAX, although as Snir mentioned would like to see any upstream bugs opened to track this.

In addition, we are using a 5.6 kernel with Dax patches by default for virtiofs for both qemu-virtiofs and cloud-hypervisor. I propose we instead switch to the LTS 5.4 upstream kernel that has all the bits for virtiofs except Dax. If some user wants to use dax, they can switch to the experimental 5.6 kernel instead.

-Archana

From: Christophe de Dinechin <cdupontd at redhat.com>
Date: Friday, October 2, 2020 at 2:43 AM
To: "Dr. David Alan Gilbert" <dgilbert at redhat.com>
Cc: kata-dev <kata-dev at lists.katacontainers.io>
Subject: Re: [kata-dev] Dax enablement in 2.0




On 2 Oct 2020, at 11:24, Dr. David Alan Gilbert <dgilbert at redhat.com<mailto:dgilbert at redhat.com>> wrote:

* Fabiano Fidêncio (fidencio at redhat.com<mailto:fidencio at redhat.com>) wrote:

Snir,

Firstly, let me reply to one specific bit of your email and also add a question.

[...]


IIUC it's currently enabled by default (qemu), can it be disabled? if
not, is it planned to be configurable?

The current govmm codes only sets the "cache-size" when it's different
from zero:
https://github.com/intel/govmm/blob/master/qemu/qemu.go#L1256-L1258

So, setting virtio_fs_cache_size here
https://github.com/kata-containers/kata-containers/blob/2.0-dev/src/runtime/cli/config/configuration-qemu-virtiofs.toml.in#L113-L114
disables DAX.

A question I'd like to add to Snir's question is ...
Wouldn't it be safer to have `virtio_fs_cache_size = 0` as a default
value when using virtio-fs, at least till DAX gets solid?

I think that should work; the current qemu vhost-user-fs.c code has:

   DEFINE_PROP_SIZE("cache-size", VHostUserFS, conf.cache_size, 0),

so ommitting the property should give us 0 and cache disabled and
ommitting the property also means you work with upstream qemu's.

I do remember in older dev branches we had the default none-0
but finally found the error of our ways.

Do you have any idea of the timeline for the DAX issues to be resolved?
2.0 is a long-lived branch, presumably, so we want to have something
in place that works when DAX is here.

You may want to look at https://bugzilla.redhat.com/show_bug.cgi?id=1876915#c30



Dave



[...]

Best Regards,
--
Fabiano Fidêncio


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--
Dr. David Alan Gilbert / dgilbert at redhat.com<mailto:dgilbert at redhat.com> / Manchester, UK


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