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Re: [RFC PATCH] Disable Book-E KVM support?

From: Bernhard Beschow
Subject: Re: [RFC PATCH] Disable Book-E KVM support?
Date: Thu, 01 Dec 2022 11:23:08 +0000

Am 1. Dezember 2022 06:06:16 UTC schrieb Nicholas Piggin <npiggin@gmail.com>:
>On Thu Dec 1, 2022 at 6:45 AM AEST, Crystal Wood wrote:
>> On Mon, 2022-11-28 at 14:36 +1000, Nicholas Piggin wrote:
>> > BookE KVM is in a deep maintenance state, I'm not sure how much testing
>> > it gets. I don't have a test setup, and it does not look like QEMU has
>> > any HV architecture enabled. It hasn't been too painful but there are
>> > some cases where it causes a bit of problem not being able to test, e.g.,
>> > 
>> > https://lists.ozlabs.org/pipermail/linuxppc-dev/2022-November/251452.html
>> > 
>> > Time to begin removal process, or are there still people using it? I'm
>> > happy to to keep making occasional patches to try keep it going if
>> > there are people testing upstream. Getting HV support into QEMU would
>> > help with long term support, not sure how big of a job that would be.
>> Not sure what you mean about QEMU not having e500 HV support?  I don't know 
>> if
>> it's bitrotted, but it's there.
>> I don't know whether anyone is still using this, but if they are, it's
>> probably e500mc and not e500v2 (which involved a bunch of hacks to get 
>> almost-
>> sorta-usable performance out of hardware not designed for virtualization).  I
>> do see that there have been a few recent patches on QEMU e500 (beyond the
>> treewide cleanup type stuff), though I don't know if they're using KVM.  
>> CCing
>> them and the QEMU list.

Thanks for CCing!

No, I'm not using KVM on e500. The goal of my patches is to run software in 
QEMU on an x86_64 host rather than on a real PPC machine to optimize our 
development process.

Best regards,

>Well I could be wrong about it, but it doesn't look it implements LPIDR
>or GSPRs. The only use of MSR_GS seems to be a couple of places
>including an instruction that aborts because no HV implementation. It
>does have an MMU index selector but before d764184ddb22 that apparently
>didn't really work.
>QEMU probably should be able to run BookE KVM in PR mode at least.
>> I have an e6500 I could occasionally test on, if it turns out people do still
>> care about this.  Don't count me as the use case, though. :-)
>Do you have a KVM setup on it? And it works with recentish upstream?
>> FWIW, as far as the RECONCILE_IRQ_STATE issue, that used to be done in
>> kvmppc_handle_exit(), but was moved in commit 9bd880a2c882 to be "cleaner and
>> faster". :-P
>Yeah that was probably reasonable at the time, that was the common way
>to do it and thie patch avoids an unnecessary expensive write to MSR
>(which my patch retains).
>I think it must have always clobbered r4 though. It's possible it wasn't
>tested with the right build option, or the right tracer active, or maybe
>the call was simple enough that it was lucky and the compiler didn't use
>r4. Easy bug to miss when it's not obvious that macro can call into C.

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