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

[Paolo Bonzini <pbonzini@redhat.com>]

On 10/10/21 21:10, Adam Lackorzynski wrote:
> Use DMA transfers in the multiboot loader to copy
> data.
> 
> This significantly lowers QEMU's startup latency by
> a factor of about 40, for example, going from 30sec
> to 0.8sec when loading modules of 120MB in size.
> This change has been used successfully for some time.
> 
> Signed-off-by: Marcus Hähnel<marcus.haehnel@kernkonzept.com>
> Signed-off-by: Adam Lackorzynski<adam@l4re.org>
> ---
>   pc-bios/multiboot.bin         | Bin 1024 -> 1536 bytes
>   pc-bios/optionrom/multiboot.S |  10 ++---
>   pc-bios/optionrom/optionrom.h |  77 ++++++++++++++++++++++++++++++++++
>   3 files changed, 82 insertions(+), 5 deletions(-)

This would break migration to QEMU 2.4 and earlier - which is in fact 
why linuxboot has a completely different option ROM when DMA is in use.

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;
  };


Paolo