On Thu, Oct 3, 2019 at 11:56 AM Stephen Boyd <swboyd@xxxxxxxxxxxx> wrote: > > Quoting Evan Green (2019-09-18 12:37:34) > > On Tue, Sep 10, 2019 at 9:09 AM Stephen Boyd <swboyd@xxxxxxxxxxxx> wrote: > > > > > > @@ -53,6 +60,9 @@ static void *try_ram_remap(resource_size_t offset, size_t size, > > > * mapping types will be attempted in the order listed below until one of > > > * them succeeds. > > > * > > > + * MEMREMAP_RO - establish a mapping whereby writes are ignored/rejected. > > > + * Attempts to map System RAM with this mapping type will fail. > > > > Why should attempts to map RAM with this flag fail? MEMREMAP_WB will > > allow RAM and quietly give you back the direct mapping, so it seems > > like at least some values in this function allow RAM. > > > > Oh, I see a comment below about "Enforce that this mapping is not > > aliasing System RAM". I guess this is worried about cache coloring? > > But is that a problem with RO mappings? I guess the RO mappings could > > get partially stale, so if the memory were being updated out from > > under you, you might see some updates but not others. Was that the > > rationale? > > Will Deacon, Dan Williams, and I talked about this RO flag at LPC and I > believe we decided to mostly get rid of the flags argument to this > function. The vast majority of callers pass MEMREMAP_WB, so I'll just > make that be the implementation default and support the flags for > encrpytion (MEMREMAP_ENC and MEMREMAP_DEC). There are a few callers that > pass MEMREMAP_WC or MEMREMAP_WT (and one that passes all of them), but I > believe those can be changed to MEMREMAP_WB and not care. There's also > the efi framebuffer code that matches the memory attributes in the EFI > memory map. I'm not sure what to do with that one to be quite honest. > Maybe EFI shouldn't care and just use whatever is already there in the > mapping? I would guess that the folks mapping things like framebuffers would care if their write-combined memory were changed to writeback. But I suppose the better authorities on that are already here, so if they think it's fine, I guess it's all good. Whatever logic is used to defend that would likely apply equally well to the EFI mappings. > > Either way, I'll introduce a memremap_ro() API that maps memory as read > only if possible and return a const void pointer as well. I'm debating > making that API fallback to memremap() if RO isn't supported for some > reason or can't work because we're remapping system memory but that > seems a little too nice when the caller could just as well decide to > fail if memory can't be mapped as read only. Sounds good. My small vote would be for the nicer fallback to memremap(). I can't think of a case where someone would rather not have their memory mapped at all than have it mapped writeable. -Evan
