I think we have three options. Here's some rough pseudo-ish-code to
sketch them out.
/* Option 1, what we have today */
/*
* This breaks if offset[i]+size[i] != offset[i+1]
* or if alignment is in play. Silly hardware breaks
* this today.
*/
for (i = 0; i < nr_xstates; i++) {
if (!enabled_xstate(i))
continue;
total_blob_size += xstate_sizes[i];
}
/* Option 2: search for the end of the last state, probably works, Ingo likes?
*/
for (i = 0; i < nr_xstates; i++) {
if (cpu_has_xsaves && !enabled_xstate(i))
continue;
end_of_state = xstate_offsets[i] + xstate_sizes[i];
if (xstate_is_aligned[i]) /* currently not implemented */
end_of_state = ALIGN(end_of_state, 64);
if (end_of_state > total_blob_size)
total_blob_size = end_of_state;
}
/* align unconditionally, maybe??? */
total_blob_size = ALIGN(total_blob_size, 64);
/* Double check our obviously bug-free math with what the CPU says */
if (!cpu_has_xsaves)
cpuid(0xD0, 0, &check_total_blob_size, ...);
else
cpuid(0xD0, 1, &check_total_blob_size, ...);
WARN_ON(check_total_blob_size != total_blob_size);
/* Option 3, trust the CPU (what Dave's patch does) */
if (!cpu_has_xsaves)
cpuid(0xD0, 0, &total_blob_size, ...);
else
cpuid(0xD0, 1, &total_blob_size, ...);
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