Re: [PATCH v5 04/27] x86/fpu/xstate: Add XSAVES system states for shadow stack

On 11/8/18 2:00 PM, Matthew Wilcox wrote:

struct a {
	char c;
	struct b b;
};

we want struct b to start at offset 8, but with __packed, it will start
at offset 1.

You're talking about how we want the struct laid out in memory if we
have control over the layout.  I'm talking about what happens if
something *else* tells us the layout, like a hardware specification
which is what is in play with the XSAVE instruction dictated layout
that's in question here.

What I'm concerned about is a structure like this:

struct foo {
        u32 i1;
        u64 i2;
};

If we leave that to natural alignment, we end up with a 16-byte
structure laid out like this:

	0-3	i1
	3-8	alignment gap
	8-15	i2

Which isn't what we want.  We want a 12-byte structure, laid out like this:

	0-3	i1
	4-11	i2

Which we get with:


struct foo {
        u32 i1;
        u64 i2;
} __packed;

Now, looking at Yu-cheng's specific example, it doesn't matter.  We've
got 64-bit types and natural 64-bit alignment.  Without __packed, we
need to look out for natural alignment screwing us up.  With __packed,
it just does what it *looks* like it does.