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    SubjectRe: [PATCH Part2 v6 10/49] x86/fault: Add support to dump RMP entry on fault
    On Mon, Jun 20, 2022 at 11:03:58PM +0000, Ashish Kalra wrote:
    > From: Brijesh Singh <brijesh.singh@amd.com>
    >
    > When SEV-SNP is enabled globally, a write from the host goes through the
    > RMP check. If the hardware encounters the check failure, then it raises
    > the #PF (with RMP set). Dump the RMP entry at the faulting pfn to help
    > the debug.
    >
    > Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
    > ---
    > arch/x86/include/asm/sev.h | 7 +++++++
    > arch/x86/kernel/sev.c | 43 ++++++++++++++++++++++++++++++++++++++
    > arch/x86/mm/fault.c | 17 +++++++++++----
    > include/linux/sev.h | 2 ++
    > 4 files changed, 65 insertions(+), 4 deletions(-)
    >
    > diff --git a/arch/x86/include/asm/sev.h b/arch/x86/include/asm/sev.h
    > index 6ab872311544..c0c4df817159 100644
    > --- a/arch/x86/include/asm/sev.h
    > +++ b/arch/x86/include/asm/sev.h
    > @@ -113,6 +113,11 @@ struct __packed rmpentry {
    >
    > #define rmpentry_assigned(x) ((x)->info.assigned)
    > #define rmpentry_pagesize(x) ((x)->info.pagesize)
    > +#define rmpentry_vmsa(x) ((x)->info.vmsa)
    > +#define rmpentry_asid(x) ((x)->info.asid)
    > +#define rmpentry_validated(x) ((x)->info.validated)
    > +#define rmpentry_gpa(x) ((unsigned long)(x)->info.gpa)
    > +#define rmpentry_immutable(x) ((x)->info.immutable)

    If you're going to do that, use inline functions pls so that it checks
    the argument at least.

    Also, add such functions only when they're called multiple times - no
    need to add one for every field if you're going to access that field
    only once in the whole kernel.

    >
    > #define RMPADJUST_VMSA_PAGE_BIT BIT(16)
    >
    > @@ -205,6 +210,7 @@ void snp_set_wakeup_secondary_cpu(void);
    > bool snp_init(struct boot_params *bp);
    > void snp_abort(void);
    > int snp_issue_guest_request(u64 exit_code, struct snp_req_data *input, unsigned long *fw_err);
    > +void dump_rmpentry(u64 pfn);
    > #else
    > static inline void sev_es_ist_enter(struct pt_regs *regs) { }
    > static inline void sev_es_ist_exit(void) { }
    > @@ -229,6 +235,7 @@ static inline int snp_issue_guest_request(u64 exit_code, struct snp_req_data *in
    > {
    > return -ENOTTY;
    > }
    > +static inline void dump_rmpentry(u64 pfn) {}
    > #endif
    >
    > #endif
    > diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
    > index 734cddd837f5..6640a639fffc 100644
    > --- a/arch/x86/kernel/sev.c
    > +++ b/arch/x86/kernel/sev.c
    > @@ -2414,6 +2414,49 @@ static struct rmpentry *__snp_lookup_rmpentry(u64 pfn, int *level)
    > return entry;
    > }
    >
    > +void dump_rmpentry(u64 pfn)

    External function - it better belong to a namespace:

    sev_dump_rmpentry()

    > +{
    > + unsigned long pfn_end;
    > + struct rmpentry *e;
    > + int level;
    > +
    > + e = __snp_lookup_rmpentry(pfn, &level);
    > + if (!e) {
    > + pr_alert("failed to read RMP entry pfn 0x%llx\n", pfn);

    Why alert?

    Dumping stuff is either pr_debug or pr_info...

    > + return;
    > + }
    > +
    > + if (rmpentry_assigned(e)) {
    > + pr_alert("RMPEntry paddr 0x%llx [assigned=%d immutable=%d pagesize=%d gpa=0x%lx"
    > + " asid=%d vmsa=%d validated=%d]\n", pfn << PAGE_SHIFT,
    > + rmpentry_assigned(e), rmpentry_immutable(e), rmpentry_pagesize(e),
    > + rmpentry_gpa(e), rmpentry_asid(e), rmpentry_vmsa(e),
    > + rmpentry_validated(e));
    > + return;
    > + }
    > +
    > + /*
    > + * If the RMP entry at the faulting pfn was not assigned, then we do not
    Who's "we"?

    > + * know what caused the RMP violation. To get some useful debug information,
    > + * let iterate through the entire 2MB region, and dump the RMP entries if
    > + * one of the bit in the RMP entry is set.
    > + */
    > + pfn = pfn & ~(PTRS_PER_PMD - 1);
    > + pfn_end = pfn + PTRS_PER_PMD;
    > +
    > + while (pfn < pfn_end) {
    > + e = __snp_lookup_rmpentry(pfn, &level);
    > + if (!e)
    > + return;
    > +
    > + if (e->low || e->high)

    This is going to confuse people because they're going to miss a zero
    entry. Just dump the whole thing.

    ...

    > @@ -579,7 +588,7 @@ show_fault_oops(struct pt_regs *regs, unsigned long error_code, unsigned long ad
    > show_ldttss(&gdt, "TR", tr);
    > }
    >
    > - dump_pagetable(address);
    > + dump_pagetable(address, error_code & X86_PF_RMP);

    Eww.

    I'd prefer to see

    pfn = dump_pagetable(address);

    if (error_code & X86_PF_RMP)
    sev_dump_rmpentry(pfn);
    instead of passing around this SEV-specific arg in generic x86 fault code.

    The change to return the pfn from dump_pagetable() should be a pre-patch ofc.

    Thx.

    --
    Regards/Gruss,
    Boris.

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

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