On Thu, Mar 24, 2022, Manali Shukla wrote:
> Current implementation of nested page table does the page
> table build up statistically with 2048 PTEs and one pml4 entry.
> That is why current implementation is not extensible.
>
> New implementation does page table build up dynamically based
> on the RAM size of the VM which enables us to have separate
> memory range to test various npt test cases.
>
> Signed-off-by: Manali Shukla <manali.shukla@xxxxxxx>
> ---
> x86/svm.c | 163 ++++++++++++++++++++++++++++++++++----------------
Ok, so I got fairly far into reviewing this (see below, but it can be ignored)
before realizing that all this new code is nearly identical to what's in lib/x86/vm.c.
E.g. find_pte_level() and install_pte() can probably used almost verbatim.
Instead of duplicating code, can you extend vm.c to as necessary? It might not
even require any changes. I'll happily clean up vm.c in the future, e.g. to fix
the misleading nomenclature and open coded horrors, but for your purposes I think
you should be able to get away with a bare minimum of changes.
> x86/svm.h | 17 +++++-
> x86/svm_npt.c | 4 +-
> 3 files changed, 130 insertions(+), 54 deletions(-)
>
> diff --git a/x86/svm.c b/x86/svm.c
> index d0d523a..67dbe31 100644
> --- a/x86/svm.c
> +++ b/x86/svm.c
> @@ -8,6 +8,7 @@
> #include "desc.h"
> #include "msr.h"
> #include "vm.h"
> +#include "fwcfg.h"
> #include "smp.h"
> #include "types.h"
> #include "alloc_page.h"
> @@ -16,38 +17,67 @@
> #include "vmalloc.h"
>
> /* for the nested page table*/
> -u64 *pte[2048];
> -u64 *pde[4];
> -u64 *pdpe;
> u64 *pml4e;
>
> struct vmcb *vmcb;
>
> -u64 *npt_get_pte(u64 address)
> +u64* get_npt_pte(u64 *pml4,
Heh, the usual way to handle wrappers is to add underscores, i.e.
u64 *npt_get_pte(u64 address)
{
return __npt_get_pte(npt_get_pml4e(), address, 1);
}
swapping the order just results in namespacing wierdness and doesn't convey to the
reader that this is an "inner" helper.
> u64 guest_addr, int level)
Assuming guest_addr is a gpa, call it gpa to avoid ambiguity over virtual vs.
physical.
> {
> - int i1, i2;
> + int l;
> + u64 *pt = pml4, iter_pte;
Please point pointers and non-pointers on separate lines. And just "pte" for
the tmp, it's not actually used as an iterator. And with that, I have a slight
preference for page_table over pt so that it's not mistaken for pte.
> + unsigned offset;
No bare unsigned please. And "offset" is the wrong terminology, "index" or "idx"
is preferable. An offset is usually an offset in bytes, this indexes into a u64
array.
Ugh, looks like that awful name comes from PGDIR_OFFSET in lib/x86/asm/page.h.
The offset, at least in Intel SDM terminology, it specifically the last N:0 bits
of the virtual address (or guest physical) that are the offset into the physical
page, e.g. 11:0 for a 4kb page, 20:0 for a 2mb page.
> +
> + assert(level >= 1 && level <= 4);
The upper bound should be NPT_PAGE_LEVEL, or root_level (see below).
> + for(l = NPT_PAGE_LEVEL; ; --l) {
Nit, need a space after "for".
Also, can you plumb in the root level? E.g. have npt_get_pte() hardcode the
root in this case. At some point this will hopefully support 5-level NPT, at
which point hardcoding the root will require updating more code than should be
necessary.
> + offset = (guest_addr >> (((l - 1) * NPT_PGDIR_WIDTH) + 12))
> + & NPT_PGDIR_MASK;
Not your code (I think), but NPT_PGDIR_MASK is an odd name since it's common to
all. The easiest thing would be to loosely follow KVM. Actually, I think it
makes sense to grab the PT64_ stuff from KVM
#define PT64_BASE_ADDR_MASK (((1ULL << 52) - 1) & ~(u64)(PAGE_SIZE-1))
#define PT64_LEVEL_BITS 9
#define PT64_LEVEL_SHIFT(level) \
(PAGE_SHIFT + (level - 1) * PT64_LEVEL_BITS)
#define PT64_INDEX(address, level)\
(((address) >> PT64_LEVEL_SHIFT(level)) & ((1 << PT64_LEVEL_BITS) - 1))
and then use those instead of having dedicated NPT_* defines. That makes it more
obvious that (a) SVM/NPT tests are 64-bit only and (b) there's nothing special
about NPT with respect to "legacy" 64-bit paging.
That will provide a nice macro, PT64_INDEX, to replace the open coded calcuations.
> + if (l == level)
> + break;
> + if (!(iter_pte & NPT_PRESENT))
> + return false;
Return "false" works, but it's all kinds of wrong. This should either assert or
return NULL.
> + pt = (u64*)(iter_pte & PT_ADDR_MASK);
> + }
> + offset = (guest_addr >> (((l - 1) * NPT_PGDIR_WIDTH) + 12))
> + & NPT_PGDIR_MASK;
Hmm, this is unnecessary because the for-loop can't terminate on its own, it
can only exit on "l == level", and offset is already correct in that case.
