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    SubjectRe: [PATCH Part2 v5 06/45] x86/sev: Invalid pages from direct map when adding it to RMP table
    On Fri, Aug 20, 2021 at 10:58:39AM -0500, Brijesh Singh wrote:
    > Subject: Re: [PATCH Part2 v5 06/45] x86/sev: Invalid pages from direct map when adding it to RMP table

    That subject needs to have a verb. I think that verb should be
    "Invalidate".

    > The integrity guarantee of SEV-SNP is enforced through the RMP table.
    > The RMP is used with standard x86 and IOMMU page tables to enforce memory
    > restrictions and page access rights. The RMP check is enforced as soon as
    > SEV-SNP is enabled globally in the system. When hardware encounters an
    > RMP checks failure, it raises a page-fault exception.
    >
    > The rmp_make_private() and rmp_make_shared() helpers are used to add
    > or remove the pages from the RMP table.

    > Improve the rmp_make_private() to
    > invalid state so that pages cannot be used in the direct-map after its
    > added in the RMP table, and restore to its default valid permission after
    > the pages are removed from the RMP table.

    That sentence needs rewriting into proper english.

    The more important thing is, though, this doesn't talk about *why*
    you're doing this: you want to remove pages from the direct map when
    they're in the RMP table because something might modify the page and
    then the RMP check will fail?

    Also, set_direct_map_invalid_noflush() simply clears the Present and RW
    bit of a pte.

    So what's up?

    > Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
    > ---
    > arch/x86/kernel/sev.c | 61 ++++++++++++++++++++++++++++++++++++++++++-
    > 1 file changed, 60 insertions(+), 1 deletion(-)
    >
    > diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
    > index 8627c49666c9..bad41deb8335 100644
    > --- a/arch/x86/kernel/sev.c
    > +++ b/arch/x86/kernel/sev.c
    > @@ -2441,10 +2441,42 @@ int psmash(u64 pfn)
    > }
    > EXPORT_SYMBOL_GPL(psmash);
    >
    > +static int restore_direct_map(u64 pfn, int npages)

    restore_pages_in_direct_map()

    > +{
    > + int i, ret = 0;
    > +
    > + for (i = 0; i < npages; i++) {
    > + ret = set_direct_map_default_noflush(pfn_to_page(pfn + i));
    > + if (ret)
    > + goto cleanup;
    > + }

    So this is looping over a set of virtually contiguous pages, I presume,
    and if so, you should add a function called

    set_memory_p_rw()

    to arch/x86/mm/pat/set_memory.c which does

    return change_page_attr_set(&addr, numpages, __pgprot(_PAGE_PRESENT | _PAGE_RW), 0);

    so that you can do all pages in one go.

    > +
    > +cleanup:
    > + WARN(ret > 0, "Failed to restore direct map for pfn 0x%llx\n", pfn + i);
    > + return ret;
    > +}
    > +
    > +static int invalid_direct_map(unsigned long pfn, int npages)

    invalidate_pages_in_direct_map()

    or so.

    > +{
    > + int i, ret = 0;
    > +
    > + for (i = 0; i < npages; i++) {
    > + ret = set_direct_map_invalid_noflush(pfn_to_page(pfn + i));

    Same as above but that helper should do the reverse:

    set_memory_np_ro()
    {
    return change_page_attr_clear(&addr, numpages, __pgprot(_PAGE_PRESENT | _PAGE_RW), 0);
    }

    Btw, please add those helpers in a separate patch.

    Thx.

    --
    Regards/Gruss,
    Boris.

    https://people.kernel.org/tglx/notes-about-netiquette

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