linux-kernel - Re: QEMU's Hyper-V HV_X64_MSR_EOM is broken with split IRQCHIP

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Message-ID: <2fd13fdb-1d62-4d76-a8a0-c63fb0575fb7@maciej.szmigiero.name>
Date: Tue, 4 Mar 2025 14:50:40 +0100
From: "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
To: Vitaly Kuznetsov <vkuznets@...hat.com>,
 Sean Christopherson <seanjc@...gle.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
 Paolo Bonzini <pbonzini@...hat.com>, Peter Xu <peterx@...hat.com>
Subject: Re: QEMU's Hyper-V HV_X64_MSR_EOM is broken with split IRQCHIP

On 4.03.2025 13:59, Vitaly Kuznetsov wrote:
> Sean Christopherson <seanjc@...gle.com> writes:
> 
>> FYI, QEMU's Hyper-V emulation of HV_X64_MSR_EOM has been broken since QEMU commit
>> c82d9d43ed ("KVM: Kick resamplefd for split kernel irqchip"), as nothing in KVM
>> will forward the EOM notification to userspace.  I have no idea if anything in
>> QEMU besides hyperv_testdev.c cares.
> 
> The only VMBus device in QEMU besides the testdev seems to be Hyper-V
> ballooning driver, Cc: Maciej to check whether it's a real problem for
> it or not.

I just did a quick check on a hv-balloon Windows 2019 setup that I had on hand
and it seems to work with "kernel-irqchip=split" the same correct way as without
this option (which AFAIK is kernel-irqchip=on for q35 machine type).

So at least this Windows version and its VMBus client driver seem to not be
affected by this issue.

Will try to look at this deeper in coming time as I am fairly busy right now
with QEMU live migration stuff before the code freeze soon.

Thanks,
Maciej