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Re: Fast-track extension proposal for "Sv32 Svnapot and Svpbmt"

Greg Favor
 

On Tue, Jul 26, 2022 at 6:38 PM Guo Ren <guoren@...> wrote:
> My earlier comments were wrt PBMTE=0 and that these bits are not ignored but instead cause a Page Fault.  Or did I miss that your proposal defines that bits [30:29] go back to being normal PA bits (and not reserved bits as in RV64)?  In which case these bits are still not ignored.
When PBMTE=0, the [31:29] are treated as PA bits.
When PBMTE=1, the [31] is N-bit and the [30:29] is Svpbmt-bits. Even
if there is no svnapot supported, software needs to keep N-bit zero.

In the current arch, the 'N' bit exists only as a function of whether the Svnapot extension is implemented or not.  Otherwise it is a Reserved bit.  And all this is orthogonal to bits [30:29] and whether Svpbmt is implemented or not.

Unless there is a good justifying reason to do differently in your RV32 proposal, bit [31] and bits [30:29] should not be mixed together in any manner.  Their arch behaviors are orthogonal to and independent of each other.

Greg

 
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