On 3/1/22 6:01 AM, Russell King (Oracle) wrote: > On Tue, Mar 01, 2022 at 05:30:41AM +0530, Anshuman Khandual wrote: >> On 2/28/22 4:27 PM, Russell King (Oracle) wrote: >>> On Mon, Feb 28, 2022 at 04:17:32PM +0530, Anshuman Khandual wrote: >>>> This defines and exports a platform specific custom vm_get_page_prot() via >>>> subscribing ARCH_HAS_VM_GET_PAGE_PROT. Subsequently all __SXXX and __PXXX >>>> macros can be dropped which are no longer needed. >>> What I would really like to know is why having to run _code_ to work out >>> what the page protections need to be is better than looking it up in a >>> table. >>> >>> Not only is this more expensive in terms of CPU cycles, it also brings >>> additional code size with it. >>> >>> I'm struggling to see what the benefit is. >> Currently vm_get_page_prot() is also being _run_ to fetch required page >> protection values. Although that is being run in the core MM and from a >> platform perspective __SXXX, __PXXX are just being exported for a table. >> Looking it up in a table (and applying more constructs there after) is >> not much different than a clean switch case statement in terms of CPU >> usage. So this is not more expensive in terms of CPU cycles. > I disagree. > > However, let's base this disagreement on some evidence. Here is the > present 32-bit ARM implementation: > > 00000048 <vm_get_page_prot>: > 48: e200000f and r0, r0, #15 > 4c: e3003000 movw r3, #0 > 4c: R_ARM_MOVW_ABS_NC .LANCHOR1 > 50: e3403000 movt r3, #0 > 50: R_ARM_MOVT_ABS .LANCHOR1 > 54: e7930100 ldr r0, [r3, r0, lsl #2] > 58: e12fff1e bx lr > > That is five instructions long. > > Please show that your new implementation is not more expensive on > 32-bit ARM. Please do so by building a 32-bit kernel, and providing > the disassembly. > > I think you will find way more than five instructions in your version - > the compiler will have to issue code to decode the protection bits, > probably using a table of branches or absolute PC values, or possibly > the worst case of using multiple comparisons and branches. It then has > to load constants that may be moved using movw on ARMv7, but on > older architectures would have to be created from multiple instructions > or loaded from the literal pool. Then there'll be instructions to load > the address of "user_pgprot", retrieve its value, and bitwise or that. > > Therefore, I fail to see how your approach of getting rid of the table > is somehow "better" than what we currently have in terms of the effect > on the resulting code. > > If you don't like the __P and __S stuff and two arch_* hooks, you could > move the table into arch code along with vm_get_page_prot() without the > additional unnecessary hooks, while keeping all the benefits of the > table lookup. Okay, will change the arm's vm_get_page_prot() implementation as suggested.
