Re: [PATCH] clockevents/drivers/i8253: Do not zero timer counter in shutdown
From: David Woodhouse <dwmw2@infradead.org>
Date: 2024-08-13 06:40:14
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On 13 August 2024 00:59:40 BST, Sean Christopherson [off-list ref] wrote:
On Fri, Aug 02, 2024, David Woodhouse wrote:quoted
On Fri, 2024-08-02 at 07:55 -0700, Sean Christopherson wrote:quoted
On Fri, Aug 02, 2024, David Woodhouse wrote:quoted
On Thu, 2024-08-01 at 20:54 +0200, Thomas Gleixner wrote:quoted
On Thu, Aug 01 2024 at 16:14, Michael Kelley wrote:quoted
I don't have a convenient way to test my sequence on KVM.But still fails in KVMBy KVM you mean the in-kernel one that we want to kill because everyone should be using userspace IRQ chips these days?What exactly do you want to kill? In-kernel local APIC obviously needs to stay for APICv/AVIC.The legacy PIT, PIC and I/O APIC.quoted
And IMO, encouraging userspace I/O APIC emulation is a net negative for KVM and the community as a whole, as the number of VMMs in use these days results in a decent amount of duplicated work in userspace VMMs, especially when accounting for hardware and software quirks.I don't particularly care, but I thought the general trend was towards split irqchip mode, with the local APIC in-kernel but i8259 PIC and I/O APIC (and the i8254 PIT, which was the topic of this discussion) being done in userspace.Yeah, that's where most everyone is headed, if not already there. Letting the I/O APIC live in userspace is probably the right direction long term, I just don't love that every VMM seems to have it's own slightly different version. But I think the answer to that is to build a library for (legacy?) device emulation so that VMMs can link to an implementation instead of copy+pasting from somwhere else and inevitably ending up with code that's frozen in time.
Some would say the right answer is to present a micro-vm machine model that doesn't have any of that crap at all. Sadly we're going in the wrong direction. For >255 vCPUs on AMD machines it looks like we even have to emulate a full virtual IOMMU with DMA translation support. Well done, AMD! (Linux is OK with the 15-bit Extended Destination ID, but not Windows)