On 04/19/2010 05:50 PM, Glauber Costa wrote:
On Sat, Apr 17, 2010 at 09:58:26PM +0300, Avi Kivity wrote:
On 04/15/2010 09:37 PM, Glauber Costa wrote:
Since we're changing the msrs kvmclock uses, we have to communicate
that to the guest, through cpuid. We can add a new KVM_CAP to the
hypervisor, and then patch userspace to recognize it.
And if we ever add a new cpuid bit in the future, we have to do that again,
which create some complexity and delay in feature adoption.
Instead, what I'm proposing in this patch is a new capability, called
KVM_CAP_X86_CPUID_FEATURE_LIST, that returns the current feature list
currently supported by the hypervisor. If we ever want to add or remove
some feature, we only need to tweak into the HV, leaving userspace untouched.
Hm. We need to update userspace anyway, since we don't like turning
features on unconditionally (it breaks live migration into an older
kernel).
Right now, we don't have any mechanism to disable, say, kvmclock cpuid bit
at userspace.
(that's a serious bug wrt migration, btw)
But let's suppose we have: What's the difference between disabling
it in the way it is now, and disabling it with the method I am proposing?
No difference.
All this ioctl say is: "Those are the current supported stuff in this HV".
It does not mandate userspace to expose all of this to the guest. It just saves
us from the job of creating yet another CAP for every bit we plan on including.
Right. Well, creating a new CAP and creating a new FEATURE flag aren't
very different, and I'd like to avoid API churn. We have enough new
APIs due to missing or badly implemented features; I'd like to avoid new
ones whenever possible.
It's not like it saves userspace anything, it has to accommodate older
kernels anyhow. We are able to ignore pre 2.6.27 kernels, but now with
kvm shipped in long term support distributions, deprecating APIs will be
much harder.
If we want to be conservative, we can keep everything but the things we know
already disabled, in userspace.
We definitely need to do that.
--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.
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