cxl: Add alternate MMIO error handling

[Michael Ellerman]

On Thu, 2015-23-07 at 06:43:56 UTC, Ian Munsie wrote:
> From: Ian Munsie <imunsie at au1.ibm.com>
> 
> userspace programs using cxl currently have to use two strategies for
> dealing with MMIO errors simultaneously. They have to check every read
> for a return of all Fs in case the adapter has gone away and the kernel
> has not yet noticed, and they have to deal with SIGBUS in case the
> kernel has already noticed, invalidated the mapping and marked the
> context as failed.
> 
> In order to simplify things, this patch adds an alternative approach
> where the kernel will return a page filled with Fs instead of delivering
> a SIGBUS. This allows userspace to only need to deal with one of these
> two error paths, and is intended for use in libraries that use cxl
> transparently and may not be able to safely install a signal handler.
> 
> This approach will only work if certain constraints are met. Namely, if
> the application is both reading and writing to an address in the problem
> state area it cannot assume that a non-FF read is OK, as it may just be
> reading out a value it has previously written. Further - since only one
> page is used per context a write to a given offset would be visible when
> reading the same offset from a different page in the mapping (this only
> applies within a single context, not between contexts).
> 
> An application could deal with this by e.g. making sure it also reads
> from a read-only offset after any reads to a read/write offset.
> 
> Due to these constraints, this functionality must be explicitly
> requested by userspace when starting the context by passing in the
> CXL_START_WORK_ERR_FF flag.
> 
> Signed-off-by: Ian Munsie <imunsie at au1.ibm.com>
> Acked-by: Michael Neuling <mikey at neuling.org>

Applied to powerpc next, thanks.

https://git.kernel.org/powerpc/c/d9232a3da8683cd9c985

cheers