linux-kernel - Follow up questions on LPC 2022: MC: Instant Detection of Virtual Devices

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Message-ID: <6AA2CAD2-C799-46A0-907C-03BB802C4FA1@vmware.com>
Date:   Mon, 19 Sep 2022 17:50:52 +0000
From:   Ajay Kaher <akaher@...are.com>
To:     "agraf@...raf.de" <agraf@...raf.de>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>
CC:     "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Ashwin Dayanand Kamat <kashwindayan@...are.com>,
        Alexey Makhalov <amakhalov@...are.com>,
        Srivatsa Bhat <srivatsab@...are.com>,
        Vasavi Sirnapalli <vsirnapalli@...are.com>,
        Nadav Amit <namit@...are.com>, Alexander Graf <graf@...zon.com>
Subject: Follow up questions on LPC 2022: MC: Instant Detection of Virtual
 Devices


Hi Bjorn, Alexander,

Thanks for attending and raising question during our talk on "Instant Detection of Virtual Devices".
Recoding link for reference: https://youtu.be/0mIO54oQ2zI?t=7151


Bjorn, I hope I understood your questions properly and replied. If some doubts please let us know.

Alexander, you wanted to know about following (please correct if we understood something wrong):
Why we still have ~300 read/write, even after having following progressive approaches:
- MMIO Direct Read
- Skip write to PCIe configs
- Pre-config of PCIe configs

[1] remaining writes:
As per PCI specification, needs to perform following operation to read BAR addr and size:
- Read current value from BAR
- Write all-ones to BAR
- Read new value from BAR
- Write original value back to BAR

Same applies to following configs as well: 
- BARs
- I/O Base/Limit
- Memory Base/Limit
- Prefetchable Memory Base/Limit
- Prefetchable Base Limit/Upper 32 bits
- I/O Base/Limit Upper 16 bits

These write operations couldn't be eliminated with our progressive approaches and major of remaining writes are because of these operations.


[2] remaining reads:
Before initialize MMIO, guest performs very few PCI config reads over PIO. I guess these can't be eliminated. 


--Ajay