Re: [PATCH v2] x86/PCI: Prefer MMIO over PIO on VMware hypervisor

[Wei Liu <wei.liu@kernel.org>]

On Tue, Sep 06, 2022 at 12:38:37PM +0530, Ajay Kaher wrote:
> During boot-time there are many PCI config reads, these could be performed
> either using Port IO instructions (PIO) or memory mapped I/O (MMIO).
> 
> PIO are less efficient than MMIO, they require twice as many PCI accesses
> and PIO instructions are serializing. As a result, MMIO should be preferred
> when possible over PIO.
> 
> Virtual Machine test result using VMware hypervisor
> 1 hundred thousand reads using raw_pci_read() took:
> PIO: 12.809 seconds
> MMIO: 8.517 seconds (~33.5% faster then PIO)
> 
> Currently, when these reads are performed by a virtual machine, they all
> cause a VM-exit, and therefore each one of them induces a considerable
> overhead.
> 
> This overhead can be further improved, by mapping MMIO region of virtual
> machine to memory area that holds the values that the “emulated hardware”
> is supposed to return. The memory region is mapped as "read-only” in the
> NPT/EPT, so reads from these regions would be treated as regular memory
> reads. Writes would still be trapped and emulated by the hypervisor.
> 
> Virtual Machine test result with above changes in VMware hypervisor
> 1 hundred thousand read using raw_pci_read() took:
> PIO: 12.809 seconds
> MMIO: 0.010 seconds
> 
> This helps to reduce virtual machine PCI scan and initialization time by
> ~65%. In our case it reduced to ~18 mSec from ~55 mSec.
> 
> MMIO is also faster than PIO on bare-metal systems, but due to some bugs
> with legacy hardware and the smaller gains on bare-metal, it seems prudent
> not to change bare-metal behavior.
> 
> Signed-off-by: Ajay Kaher <akaher@vmware.com>

The subject line should be fixed -- you're changing the behaviour for
all hypervisors, not just VMWare. I almost skipped this because of the
subject line.

Thanks,
Wei.