Re: [PATCH 6/7] x86/vga: fix mapping of the VGA text buffer

[Jan Beulich <jbeulich@xxxxxxxx>]

On 14.03.2025 11:39, Roger Pau Monné wrote:
> (resending because I seem to have inadvertently corrupted the Cc field)
> 
> On Thu, Mar 13, 2025 at 07:39:58PM +0000, Andrew Cooper wrote:
>> On 13/03/2025 3:30 pm, Roger Pau Monne wrote:
>>> The call to ioremap_wc() in video_init() will always fail, because
>>> video_init() is called ahead of vm_init_type(), and so the underlying
>>> __vmap() call will fail to allocate the linear address space.
>>>
>>> Fix by reverting to the previous behavior and using the directmap entries
>>> in the low 1MB.  Note the VGA text buffer directmap entries are also
>>> adjusted to map the VGA text buffer as WC instead of UC-.
>>>
>>> Fixes: 81d195c6c0e2 ('x86: introduce ioremap_wc()')
>>> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>>> ---
>>>  xen/arch/x86/boot/x86_64.S        | 10 +++++++---
>>>  xen/arch/x86/include/asm/config.h |  5 +++++
>>>  xen/drivers/video/vga.c           | 11 ++++++++---
>>>  3 files changed, 20 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/xen/arch/x86/boot/x86_64.S b/xen/arch/x86/boot/x86_64.S
>>> index 26b9d1c2df9a..07f4bdf46e31 100644
>>> --- a/xen/arch/x86/boot/x86_64.S
>>> +++ b/xen/arch/x86/boot/x86_64.S
>>> @@ -84,15 +84,19 @@ ENTRY(__high_start)
>>>  /*
>>>   * Mapping of first 2 megabytes of memory. This is mapped with 4kB mappings
>>>   * to avoid type conflicts with fixed-range MTRRs covering the lowest 
>>> megabyte
>>> - * of physical memory. In any case the VGA hole should be mapped with type 
>>> UC.
>>> + * of physical memory. VGA hole should be mapped with type UC, with the
>>> + * exception of the text buffer that uses WC.
>>>   * Uses 1x 4k page.
>>>   */
>>>  l1_directmap:
>>>          pfn = 0
>>>          .rept L1_PAGETABLE_ENTRIES
>>> -        /* VGA hole (0xa0000-0xc0000) should be mapped UC-. */
>>> -        .if pfn >= 0xa0 && pfn < 0xc0
>>> +        /* VGA hole (0xa0000-0xb8000) should be mapped UC-. */
>>> +        .if pfn >= 0xa0 && pfn < 0xb8
>>>          .quad (pfn << PAGE_SHIFT) | __PAGE_HYPERVISOR_UCMINUS | 
>>> _PAGE_GLOBAL | MAP_SMALL_PAGES
>>> +        /* VGA text buffer (0xb80000-0xc0000) should be mapped WC. */
>>> +        .elseif pfn >= 0xb8 && pfn < 0xc0
>>> +        .quad (pfn << PAGE_SHIFT) | __PAGE_HYPERVISOR_WC | _PAGE_GLOBAL | 
>>> MAP_SMALL_PAGES
>>>          .else
>>>          .quad (pfn << PAGE_SHIFT) | PAGE_HYPERVISOR_RWX | MAP_SMALL_PAGES
>>>          .endif
>>
>> We have to be careful doing this.
>>
>> It probably is safe to use WC in the pagetables.  We don't start using
>> the pagetables until after we're sure we're on a 64bit CPU, which means
>> WC is available.
>>
>> However, doing so now means that we need explicit SFENCE's when using
>> this, even in places like early_error.  The IN/OUT instructions do flush
>> WC buffers, but the UART is written to before the screen, so there's a
>> chance that you'll lose the final character of the message on the screen.
> 
> I don't think early_error will ever use this mapping.
> 
> `vga_text_buffer` contains the address 0xb8000, and AFAICT it's
> exclusively used with paging disabled (as the multiboot2 efi path
> explicitly sets vga_text_buffer = 0).  The WC mapping created above is
> on the directmap, so va > DIRECTMAP_VIRT_START.
> 
> vga_text_puts() might need such SFENCE, but arguably that should be a
> different patch IMO.  Might be best to ask Jan whether this is on
> purpose?

I think that was wrongly omitted before already.

> My hypothesis is that the SFENCE might only be needed in
> video_endboot() and before reboot if Xen crashed ahead of
> relinquishing the VGA console.

This might suffice for being able to see the final picture, but it may
result in display artifacts earlier on.

Question is whether simply undoing the ioremap_wc() (for not functioning
correctly) isn't going to be good enough. Prior to the change to use that,
we had been using UC- quite okay (even if a bit slow). An option may be
to make a WC mapping a little later, when __vmap() is usable.

Jan