Re: [PATCH v3 3/3] OvmfPkg: save on I/O port accesses when the debug port is not in use

[Laszlo Ersek <lersek@redhat.com>]

On 11/16/17 21:31, Paolo Bonzini wrote:
> When SEV is enabled, every debug message printed by OVMF to the
> QEMU debug port traps from the guest to QEMU character by character
> because "REP OUTSB" cannot be used by IoWriteFifo8.  Furthermore,
> when OVMF is built with the DEBUG_VERBOSE bit (value 0x00400000)
> enabled in "gEfiMdePkgTokenSpaceGuid.PcdDebugPrintErrorLevel", then the
> OvmfPkg/IoMmuDxe driver, and the OvmfPkg/Library/BaseMemEncryptSevLib
> library instance that is built into it, produce a huge amount of
> log messages.  Therefore, in SEV guests, the boot time impact is huge
> (about 45 seconds _additional_ time spent writing to the debug port).
> 
> While these messages are very useful for analyzing guest behavior,
> most of the time the user won't be capturing the OVMF debug log.
> In fact libvirt does not provide a method for configuring log capture;
> users that wish to do this (or are instructed to do this) have to resort
> to <qemu:arg>.
> 
> The debug console device provides a handy detection mechanism; when read,
> it returns 0xE9 (which is very much unlike the 0xFF that is returned by
> an unused port).  Use it to skip the possibly expensive OUT instructions
> when the debug I/O port isn't plugged anywhere.
> 
> For SEC, the debug port has to be read before each full message.
> However:
> 
> - if the debug port is available, then reading one byte before writing
> a full message isn't tragic, especially because SEC doesn't print many
> messages
> 
> - if the debug port is not available, then reading one byte instead of
> writing a full message is still a win.
> 
> Contributed-under: TianoCore Contribution Agreement 1.0
> Cc: Laszlo Ersek <lersek@redhat.com>
> Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> Cc: Jordan Justen (Intel address) <jordan.l.justen@intel.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  .../PlatformDebugLibIoPort/DebugLibDetect.h        | 57 ++++++++++++++++++++++
>  OvmfPkg/Library/PlatformDebugLibIoPort/DebugLib.c  | 28 +++++++++--
>  .../PlatformDebugLibIoPort/DebugLibDetect.c        | 30 ++++++++++--
>  .../PlatformDebugLibIoPort/DebugLibDetectRom.c     | 21 +++++++-
>  4 files changed, 127 insertions(+), 9 deletions(-)
>  create mode 100644 OvmfPkg/Library/PlatformDebugLibIoPort/DebugLibDetect.h

(looks like you didn't pick up my T-b from the v2 review for this patch;
I'm adding that back in)