On 01/29/2013 01:12 PM, Eric Blake wrote:
On 01/29/2013 06:41 AM, John Ferlan wrote:
*(pm + (j / 8)) |= cpumap[j] << (8 * (j & 7));
FWIW: This path of code only occurs when "(hv_versions.hypervisor <= 1)" - IOW really old code. I checked history and the code was added by commit id '86247f2c'. Also since the target of the "|" operation is a 'uint64_t' (e.g. *(pm + (j / 8)), wouldn't the shift from 0->56 be OK (e.g. (8 * (j & 7)))? That is it's not an 'int << (8*4)' it's a 'uint64_t << (8*4)'.
Order of operations, broken down by type:
*(uint64_t* + (int / int)) |= unsigned char << (int * (int & int)) *(uint64_t* + int) |= unsigned char << int *(uint64_t*) |= int << int uint64_t = uint64_t | int
There really is a sign extension bug, because 'unsigned char << int' uses 'int' math, but we then widen the int into uint64_t. If vcpu 31 is turned on, we end up sign-extending and also enabling vcpus 32-63 at the same time, which was not the goal.
The following silences the [sign_extension] issue: *(pm + (j / 8)) |= (uint64_t)cpumap[j] << (8 * (j & 7)); The following silences the [ptr_arith] issue: pm = (uint64_t *)((uint64_t)&xen_cpumap + (j & ~0x7UL)); *pm |= (uint64_t)cpumap[j] << (8 * (j & 7)); Of course the code from my first response last night also silences both.
When first approaching this I figured I didn't want to introduce a bug into code that's been around a long time and that may not have any one using it. I agree the line looks ugly and it did take me a bit to think about it.
Just because no one has reported the bug or tested it lately doesn't mean that the bug isn't there - it has been there a LONG time. :)
Understood. Nobody's complained either so no one's been able to fix what's not broke :-)
Mathematically what you propose with the memcpy() works; however, I come from an architecture where a memcpy to an unaligned address causes a fault
Such a memcpy() implementation would be flawed, and should be reported as a bug to that vendor; thankfully, these days, most libc do not have that flaw. The C standard guarantees that memcpy() is required to work on unaligned copies (for a certain definition of work; you may still end up splicing together unrelated portions of adjacent integers when reading things back as integers, but byte-for-byte, the operation is well-defined).
Alpha and Itanium have some (silent unless noise enabled) heartache when the "to" address is "0x7ffe8f2b8cf1" (or x2, x3, x5, x6, x7, etc). Ending with x0 & x8 are preferred while x4 & xc tolerated. If the noise isn't turned on, the application performs poorly due to excessive alignment faults. John