Re: [PATCH] multiboot: Use DMA instead port-based transfer

[Marcus Hähnel]

On Tuesday, October 19, 2021 6:45:44 PM CEST Paolo Bonzini wrote:
> On my system (a relatively recent laptop) I get 15-20 MiB per second, 
> which is slow but not as slow as what you got.  Out of curiosity, can 
> you test what you get with the following kernel patch?
> 
> diff --git a/arch/x86/kvm/kvm_emulate.h b/arch/x86/kvm/kvm_emulate.h
> index 798508e8b6f5..5853ae93bcb2 100644
> --- a/arch/x86/kvm/kvm_emulate.h
> +++ b/arch/x86/kvm/kvm_emulate.h
> @@ -272,7 +272,7 @@ struct fetch_cache {
>   };
> 
>   struct read_cache {
> -     u8 data[1024];
> +     u8 data[4096];
>       unsigned long pos;
>       unsigned long end;
>   };

Hi Paolo,

Thank you very much for the cleaned up and improved patch in the other
thread, which solves our issue perfectly! Your work is much appreciated.

Regarding your question above I made some quick benchmark runs. Using a
195kB kernel image and measuring from QEmu start until the first complete
line is sent over the serial output I get the following timings, all 
numbers in seconds:

 kvm?  | DMA Multiboot | Old Multiboot |
-------|---------------|---------------|
no-kvm |  0.209 ± 0.01 | 15.283 ± 0.19 |
kvm    |  0.207 ± 0.01 | 20.771 ± 0.26 |
kvm-4k |  0.208 ± 0.01 | 19.878 ± 0.22 |

The tests were run 10 times using perf stat -r10. The table shows the
averages and standard deviation. While perf does have some overhead
the general issue is independent of that.

Changing the read cache to 4k has a negligible impact compared to running
without kvm. The numbers for DMA are roughly two orders of magnitude better
in all cases.

Hardware: Lenovo T480, Intel(R) Core(TM) i7-8550U CPU @ 1.80GHz
Software: Gentoo, Custom Linux 5.14.9 kernel
          QEmu master with your DMA multiboot patches applied, 
v6.1.0-1564-g1a510366d8-dirty
          configured without any options, only the x86_64 softmmu as target
Commandline: qemu-system-x86_64 -enable-kvm -kernel /path/to/kernel -serial 
stdio -nographic -monitor none -m 512

General ballpark of results was confirmed on a different T480 with
another OS and QEmu version and on a server system. As you can see we get
not MiB/s when booting through multiboot without DMA but KiB/s.


- Marcus

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