Re: Still concerning ...
"Meltdown is not a processor microcode issue"
that may not actually be true. In the case of meltdown, it's a serious design flaw in which a memory access is being done during 'out of order' execution, that bypasses normal tests. "At some point" the page fault or protection fault will occur, but not until AFTER the memory location was actually read in.
The problem here might be fixable with microcode if the microcode can be modified to avoid actually reading any memory location until after all "leading up to it" read operations have been access checked.
As I understand it, Meltdown crafts code to execute 'out of order' by first reading a kernel memory location into a register, and then uses that value as an index into an array. The 'out of order execution' model ends up calculating the correct memory offset (even though there should have been an access violation) and then 'hits' the memory location within the array [and you use the side-channel technique of measuring which block of your array is now 'cached' after that]. A page fault or some kind of access violation occurs too long AFTER the kernel memory was accessed, and (specifically) NOT before reading the memory location within the array indexed by the kernel memory location's byte value, which is what makes this attack possible. [the array would be specifically designed for this and deliberately flushed from all caches before doing this, one byte at a time].
Fixing this would involve determining that the initial access into kernel space was not valid, and thereby STOP the rest of the 'out of order / hyperthreading execution' stuff from happening. This assumes it's microcode, and not flawed silicon causing this. It also explains why Intel is vulnerable, and not AMD, because [apparently] the AMD people figured out that this was a BAD thing beforehand.
And then I have to wonder if there's a slight performance hit by doing this in the silicon or in the microcode, therefore implying that Intel 'cheated' by allowing the flaw to be there to ge a slight boost in speed over AMD. This last part is pure speculation of course. It could have been accidental, too.
So yeah, MAYBE Meltdown can be patched in microcode. Unless you've heard different from Intel... ?