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List:       linux-efi
Subject:    Re: [PATCH 0/2] efi: add support for cacheable efifb mappings
From:       Ard Biesheuvel <ard.biesheuvel () linaro ! org>
Date:       2018-06-18 11:49:44
Message-ID: CAKv+Gu9RTB2WFDiUKQmm2ohyA85UHpS9R0_09eDLgtsGHY6dzw () mail ! gmail ! com
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On 15 June 2018 at 18:39, Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote:
> On 15 June 2018 at 18:38, Laszlo Ersek <lersek@redhat.com> wrote:
>> On 06/15/18 12:48, Ard Biesheuvel wrote:
>>> KVM guests on ARM have special requirements when it comes to mapping
>>> framebuffers: given that the host [emulating the framebuffer] uses
>>> cacheable accesses to read from the framebuffer region, the guest
>>> should uses cacheable accesses as well, or coherency is lost, i.e.,
>>> the host does not see what the guest writes.
>>>
>>> Modifying PCI drivers to take this into account just for KVM on ARM is
>>> unreasonable, given that mapping BARs cacheable and still expecting
>>> side effects does not make any sense. However, doing the same for
>>> regions of system memory does make sense, since a framebuffer in DRAM
>>> could be accessed via DMA by a coherent master, and so it makes sense
>>> to take the memory attributes described by the UEFI memory map into
>>> account if it covers the efifb region.
>>>
>>> Patch #2 implements this. Patch #1 is a preparatory patch that makes
>>> efi_mem_desc_lookup() usable for us in #2.
>>>
>>> Question is how to best test this. I tried Gerd's ramfb patches against
>>> QEMU with a recent ArmVirtQemu build but I am having trouble getting my
>>> console to use the EFI framebuffer.
>>>
>>> Ard Biesheuvel (2):
>>>   efi: drop type and attribute checks in efi_mem_desc_lookup()
>>>   fbdev/efifb: honour UEFI memory map attributes when mapping the fb
>>>
>>>  arch/x86/platform/efi/quirks.c |  3 +-
>>>  drivers/firmware/efi/efi.c     |  8 +--
>>>  drivers/firmware/efi/esrt.c    |  5 +-
>>>  drivers/video/fbdev/efifb.c    | 52 ++++++++++++++++----
>>>  4 files changed, 49 insertions(+), 19 deletions(-)
>>>
>>
>> I reproduced the issue using the latest stock F28 kernel in the KVM
>> guest, on my Mustang A3 host. Then I built this set on top of
>> v4.17-12074-g4c5e8fc62d6a, using the F28 kernel config. I didn't pass
>> any parameters to efifb. The series works perfectly for me.
>>
>> Tested-by: Laszlo Ersek <lersek@redhat.com>
>>
>
> Thanks! Very happy to hear that ...

Peter, any objections? If not, I will resend them with Laszlo's T-b
and cc the fbdev list and maintainer to get this into v4.19

Thanks,
Ard.
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