Re: [PATCH 1/3] sparc32: honour phys_base in the viking cache flush routines

[Sam Ravnborg <sam@ravnborg.org>]

Hi Magnus.

On Fri, Aug 14, 2026 at 12:52:32PM +0200, Magnus Lindholm wrote:
> viking_flush_page() and viking_mxcc_flush_page() derive the physical
> address of the page they are asked to flush by subtracting PAGE_OFFSET
> from the kernel virtual address:
> 
> 	sethi	%hi(PAGE_OFFSET), %g2
> 	sub	%o0, %g2, %g3
> 
> That is only the physical address when phys_base is zero. The C side spells
> the same conversion __pa(), which adds phys_base, and every caller passes a
> kernel virtual address expecting exactly that.
> 
> With a kernel loaded away from the start of RAM the two disagree by
> phys_base. viking_flush_page() then compares cache tags against the wrong
> page and flushes nothing, and viking_mxcc_flush_page() streams a page that
> is phys_base lower than the one it was given, so the intended lines stay
> dirty in the cache while unrelated ones are pushed out.
> 
> The visible effect is that anything relying on a flush to make memory
> visible to another bus master silently keeps working from stale data. On a
> SPARCstation 20 this shows up as every SCSI transfer failing with a DMA
> error: iommu_flush_iotlb() cannot get the IOPTEs out to RAM, so the IOMMU
> walks stale entries and the ESP DMA faults.
> 
> Add phys_base, so these agree with __pa() again. No change when phys_base
> is zero, which is why this went unnoticed.
> 
> Signed-off-by: Magnus Lindholm <linmag7@gmail.com>
Looks good, I checked and as the other variants are coherent this
should be the only place we need this.

Reviewed-by: Sam Ravnborg <sam@ravnborg.org>