Re: [PATCH v6 6/8] x86/sev: Treat the contiguous RMP table as a single RMP segment

[Borislav Petkov <bp@alien8.de>]

On Mon, Dec 09, 2024 at 08:43:20PM +0100, Borislav Petkov wrote:
> I think we should collect the deliberation why we're doing this nospec stuff
> from here:
> 
> https://lore.kernel.org/r/79be2e29-6487-dd60-9b6f-3daa48a2e93f@amd.com
> 
> into a comment around here for future reference.
> 
> This is the best example for those: "uff, why did we do it back then"?
> questions when we stare at this months, years from now.

IOW, something short like this which should catch the gist of the idea:

diff --git a/arch/x86/virt/svm/sev.c b/arch/x86/virt/svm/sev.c
index 3272a73d3594..46abc73cbb87 100644
--- a/arch/x86/virt/svm/sev.c
+++ b/arch/x86/virt/svm/sev.c
@@ -453,6 +453,14 @@ bool snp_probe_rmptable_info(void)
        return true;
 }
 
+/*
+ * About the array_index_nospec() usage below:
+ *
+ * This function can get called by exported functions like
+ * snp_lookup_rmpentry(), which is used by the KVM #PF handler, among
+ * others, and since the @pfn passed in cannot always be trusted,
+ * speculation should be stopped as a protective measure.
+ */
 static struct rmpentry_raw *get_raw_rmpentry(u64 pfn)
 {
        u64 paddr, rst_index, segment_index;

-- 
Regards/Gruss,
    Boris.

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