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From: | Paolo Bonzini |
Subject: | Re: [Qemu-devel] [PATCH RFC] kvm: ignore apic polarity |
Date: | Fri, 28 Feb 2014 00:31:27 +0100 |
User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.2.0 |
Il 28/02/2014 00:13, Gabriel L. Somlo ha scritto:
On Thu, Feb 27, 2014 at 11:30:55PM +0100, Paolo Bonzini wrote:Il 27/02/2014 22:41, Gabriel L. Somlo ha scritto:On Thu, Feb 27, 2014 at 07:05:49PM +0200, Michael S. Tsirkin wrote:apic polarity in KVM does not work: too many things assume active high. Let's not pretend it works, let's just ignore polarity flag. If we ever want to emulate it exactly, this will need a feature flag anyway. Also report this to userspace: this makes it possible to report the interrupt active-low in ACPI, this way we are closer to real hardware. This patch fixes OSX running on KVM. Reported-by: "Gabriel L. Somlo" <address@hidden> Signed-off-by: Michael S. Tsirkin <address@hidden> ---So, the way I understand this (and I'm writing this mainly for myself, to make sure I understand correctly, so please kick me if I got it wrong), ACPI tells the guest OS how to configure "physical" ioapic polarity. With ActiveHigh, "physical" == "logical", i.e. "high" == "asserted" and "low" == "deasserted". With ActiveLow, "physical" == !"logical", so the other way around. QEMU being hard-coded to ActiveHigh is the moral equivalent of always sending the kernel (KVM) "logical" line states, rather than "physical" ones. Assuming KVM's userland clients are all coded for ActiveHigh, we can (should, for sanity's sake) just assume line states from userland are logical, and stop paying attention the polarity bits. That way, misbehaving guests [*] can configure their ioapics as they please, and things will just work OK regardless. As you pointed out earlier, even KVM itself already kind-of assumes ActiveHigh (e.g. in __kvm_irq_line_state(), which should be coded differently if ActiveLow were a serious possibility, and, BTW, irq_states[irq] would probably have to be initialized to all-1's if ActiveLow wre used, etc, etc).This is a much better description. Can you turn it into a patch to Documentation/virtual/kvm/api.txt and a more complete commit message?Do you mean one patch to change both virt/kvm/ioapic.c and Documentation/virtual/kvm/api.txt ? Or a separate documentation patch ? (sorry for my ignorance, I'm new to being a KVM contributor :) )
Yes.
I think removing the polarity xor from KVM is about giving up on trying to add ActiveLow support to QEMU altogether. What I tested was what would happen if Linux (which pays attention to ACPI) were told to use ActiveLow, but thre rest of QEMU continued being hardcoded as ActiveHigh. Basically, another datapoint similar to what happens with OS X, which completely ignores ACPI and configures the ioapic as ActiveLow (even while running on ActiveHigh "hardware", i.e. QEMU). With KVM no longer paying attention to the polarity bit, things work fine, both with Linux-thinking-it's-ActiveLow, and with OS X. But, since QEMU will stay ActiveHigh, I don't think TCG will be impacted in any way by this change.
If you change ACPI tables to ActiveLow, and leave the polarity inversion in hw/intc/ioapic.c, then it will matter.
(Hmmm, maybe this one of the reasons I never got OS X to boot without -enable-kvm... I should look at the QEMU hw/intc/ioapic*.c, and see if *it* cares about guest-configured polarity, and maybe get it to *stop* caring :)
Exactly my point. :) Paolo
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