Re: Patch to align virt and phys spaces on i386

On Wed, 8 Mar 2006, Glenn Brown wrote:
> The attached patch increases i386 virt_offset alignment to 4kB, so the 
> bottom 12 bits of corresponding virt and bus addrs will be identical.

Good idea for Etherboot as-is, since it removes the need for drivers to 
care about aligning physical addresses, but won't work in the "stay in 
real mode" (aka KEEP_IT_REAL) port in Etherboot 5.5, since the PXE spec 
guarantees us only 16-byte alignment in where we get loaded, and we don't 
get to relocate ourselves.  We therefore need another mechanism to deal 
with buffer alignment anyway, and I'd prefer to keep it down to just one 
mechanism for simplicity.

I accept that the current approach, in which each driver allocates more 
space than it needs and then slides its buffers to align them, is 
inefficient and duplicative.  In 5.5, all data buffers are part of the 
shared buffer space, so maybe we could implement a mechanism whereby we 
refer to this space via a global pointer that gets maximally aligned by 
shared (non-driver) code.  This would give drivers aligned buffers while 
removing the duplicate code and maintaining the option of PXE compliance.

Michael





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