Oh, here's the THP code. The subject just changed.
On 7/7/21 11:35 AM, Brijesh Singh wrote:
> When SEV-SNP is enabled globally, a write from the host goes through the
> RMP check. When the host writes to pages, hardware checks the following
> conditions at the end of page walk:
>
> 1. Assigned bit in the RMP table is zero (i.e page is shared).
> 2. If the page table entry that gives the sPA indicates that the target
> page size is a large page, then all RMP entries for the 4KB
> constituting pages of the target must have the assigned bit 0.
> 3. Immutable bit in the RMP table is not zero.
>
> The hardware will raise page fault if one of the above conditions is not
> met. Try resolving the fault instead of taking fault again and again. If
> the host attempts to write to the guest private memory then send the
> SIGBUG signal to kill the process. If the page level between the host and
"SIGBUG"?
> RMP entry does not match, then split the address to keep the RMP and host
> page levels in sync.
> ---
> arch/x86/mm/fault.c | 69 +++++++++++++++++++++++++++++++++++++++++++++
> include/linux/mm.h | 6 +++-
> mm/memory.c | 13 +++++++++
> 3 files changed, 87 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index 195149eae9b6..cdf48019c1a7 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -1281,6 +1281,58 @@ do_kern_addr_fault(struct pt_regs *regs, unsigned long hw_error_code,
> }
> NOKPROBE_SYMBOL(do_kern_addr_fault);
>
> +#define RMP_FAULT_RETRY 0
> +#define RMP_FAULT_KILL 1
> +#define RMP_FAULT_PAGE_SPLIT 2
> +
> +static inline size_t pages_per_hpage(int level)
> +{
> + return page_level_size(level) / PAGE_SIZE;
> +}
> +
> +static int handle_user_rmp_page_fault(unsigned long hw_error_code, unsigned long address)
> +{
> + unsigned long pfn, mask;
> + int rmp_level, level;
> + struct rmpentry *e;
> + pte_t *pte;
> +
> + if (unlikely(!cpu_feature_enabled(X86_FEATURE_SEV_SNP)))
> + return RMP_FAULT_KILL;
Shouldn't this be a WARN_ON_ONCE()? How can we get RMP faults without
SEV-SNP?
> + /* Get the native page level */
> + pte = lookup_address_in_mm(current->mm, address, &level);
> + if (unlikely(!pte))
> + return RMP_FAULT_KILL;
What would this mean? There was an RMP fault on a non-present page?
How could that happen? What if there was a race between an unmapping
event and the RMP fault delivery?
> + pfn = pte_pfn(*pte);
> + if (level > PG_LEVEL_4K) {
> + mask = pages_per_hpage(level) - pages_per_hpage(level - 1);
> + pfn |= (address >> PAGE_SHIFT) & mask;
> + }
This looks inherently racy. What happens if there are two parallel RMP
faults on the same 2M page. One of them splits the page tables, the
other gets a fault for an already-split page table.
Is that handled here somehow?
> + /* Get the page level from the RMP entry. */
> + e = snp_lookup_page_in_rmptable(pfn_to_page(pfn), &rmp_level);
> + if (!e)
> + return RMP_FAULT_KILL;
The snp_lookup_page_in_rmptable() failure cases looks WARN-worthly.
Either you're doing a lookup for something not *IN* the RMP table, or
you don't support SEV-SNP, in which case you shouldn't be in this code
in the first place.
> + /*
> + * Check if the RMP violation is due to the guest private page access.
> + * We can not resolve this RMP fault, ask to kill the guest.
> + */
> + if (rmpentry_assigned(e))
> + return RMP_FAULT_KILL;
No "We's", please. Speak in imperative voice.
> + /*
> + * The backing page level is higher than the RMP page level, request
> + * to split the page.
> + */
> + if (level > rmp_level)
> + return RMP_FAULT_PAGE_SPLIT;
This can theoretically trigger on a hugetlbfs page. Right?
I thought I asked about this before... more below...
> + return RMP_FAULT_RETRY;
> +}
> +
> /*
> * Handle faults in the user portion of the address space. Nothing in here
> * should check X86_PF_USER without a specific justification: for almost
> @@ -1298,6 +1350,7 @@ void do_user_addr_fault(struct pt_regs *regs,
> struct task_struct *tsk;
> struct mm_struct *mm;
> vm_fault_t fault;
> + int ret;
> unsigned int flags = FAULT_FLAG_DEFAULT;
>
> tsk = current;
> @@ -1378,6 +1431,22 @@ void
(struct pt_regs *regs,
> if (error_code & X86_PF_INSTR)
> flags |= FAULT_FLAG_INSTRUCTION;
>
> + /*
> + * If its an RMP violation, try resolving it.
> + */
> + if (error_code & X86_PF_RMP) {
> + ret = handle_user_rmp_page_fault(error_code, address);
> + if (ret == RMP_FAULT_PAGE_SPLIT) {
> + flags |= FAULT_FLAG_PAGE_SPLIT;
> + } else if (ret == RMP_FAULT_KILL) {
> + fault |= VM_FAULT_SIGBUS;
> + do_sigbus(regs, error_code, address, fault);
> + return;
> + } else {
> + return;
> + }
> + }
Why not just have handle_user_rmp_page_fault() return a VM_FAULT_* code
directly?
I also suspect you can just set VM_FAULT_SIGBUS and let the do_sigbus()
call later on in the function do its work.
> * Faults in the vsyscall page might need emulation. The
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 322ec61d0da7..211dfe5d3b1d 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -450,6 +450,8 @@ extern pgprot_t protection_map[16];
> * @FAULT_FLAG_REMOTE: The fault is not for current task/mm.
> * @FAULT_FLAG_INSTRUCTION: The fault was during an instruction fetch.
> * @FAULT_FLAG_INTERRUPTIBLE: The fault can be interrupted by non-fatal signals.
> + * @FAULT_FLAG_PAGE_SPLIT: The fault was due page size mismatch, split the
> + * region to smaller page size and retry.
> *
> * About @FAULT_FLAG_ALLOW_RETRY and @FAULT_FLAG_TRIED: we can specify
> * whether we would allow page faults to retry by specifying these two
> @@ -481,6 +483,7 @@ enum fault_flag {
> FAULT_FLAG_REMOTE = 1 << 7,
> FAULT_FLAG_INSTRUCTION = 1 << 8,
> FAULT_FLAG_INTERRUPTIBLE = 1 << 9,
> + FAULT_FLAG_PAGE_SPLIT = 1 << 10,
> };
>
> /*
> @@ -520,7 +523,8 @@ static inline bool fault_flag_allow_retry_first(enum fault_flag flags)
> { FAULT_FLAG_USER, "USER" }, \
> { FAULT_FLAG_REMOTE, "REMOTE" }, \
> { FAULT_FLAG_INSTRUCTION, "INSTRUCTION" }, \
> - { FAULT_FLAG_INTERRUPTIBLE, "INTERRUPTIBLE" }
> + { FAULT_FLAG_INTERRUPTIBLE, "INTERRUPTIBLE" }, \
> + { FAULT_FLAG_PAGE_SPLIT, "PAGESPLIT" }
>
> /*
> * vm_fault is filled by the pagefault handler and passed to the vma's
> diff --git a/mm/memory.c b/mm/memory.c
> index 730daa00952b..aef261d94e33 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4407,6 +4407,15 @@ static vm_fault_t handle_pte_fault(struct vm_fault *vmf)
> return 0;
> }
>
> +static int handle_split_page_fault(struct vm_fault *vmf)
> +{
> + if (!IS_ENABLED(CONFIG_AMD_MEM_ENCRYPT))
> + return VM_FAULT_SIGBUS;
> +
> + __split_huge_pmd(vmf->vma, vmf->pmd, vmf->address, false, NULL);
> + return 0;
> +}
What will this do when you hand it a hugetlbfs page?
