Subject: Re: [PATCH] CXL: Fix PSL error due to duplicate segment table entries

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Mon, 27 Oct 2014 20:08:41 +0530

Ian Munsie <[email-masked]> writes:

> From: Ian Munsie <[email-masked]>
>
> In certain circumstances the PSL can send an interrupt for a segment
> miss that the kernel has already handled. This can happen if multiple
> translations for the same segment are queued in the PSL before the
> kernel has restarted the first translation.
>
> The CXL driver did not expect this situation and did not check if a
> segment had already been handled. This could cause a duplicate segment
> table entry which in turn caused a PSL error taking down the card.
>
> This patch fixes the issue by checking for existing entries in the
> segment table that match the segment it is trying to insert to avoid
> inserting duplicate entries.
>
> Some of the code has been refactored to simplify it - the segment table
> hash has been moved from cxl_load_segment to find_free_sste where it is
> used and we have disabled the secondary hash in the segment table to
> reduce the number of entries that need to be tested from 16 to 8. Due to
> the large segment sizes we use it is extremely unlikely that the
> secondary hash would ever have been used in practice, so this should not
> have any negative impacts and may even improve performance.
>
> copro_calculate_slb will now mask the ESID by the correct mask for 1T vs
> 256M segments. This has no effect by itself as the extra bits were
> ignored, but it makes debugging the segment table entries easier and
> means that we can directly compare the ESID values for duplicates
> without needing to worry about masking in the comparison.
>
> Signed-off-by: Ian Munsie <[email-masked]>

I guess you are missing too many fixes in one patch.

1) One cleanup
2) Fix for masking ea correctly
3) And fix for not erroring out when a slb is already in the slb cache.

> ---
>  arch/powerpc/mm/copro_fault.c |  3 +-
>  drivers/misc/cxl/fault.c      | 73 ++++++++++++++++++++++---------------------
>  drivers/misc/cxl/native.c     |  4 +--
>  3 files changed, 41 insertions(+), 39 deletions(-)
>
> diff --git a/arch/powerpc/mm/copro_fault.c b/arch/powerpc/mm/copro_fault.c
> index 0f9939e..5a236f0 100644
> --- a/arch/powerpc/mm/copro_fault.c
> +++ b/arch/powerpc/mm/copro_fault.c
> @@ -99,8 +99,6 @@ int copro_calculate_slb(struct mm_struct *mm, u64 ea, struct copro_slb *slb)
>  	u64 vsid;
>  	int psize, ssize;
>  
> -	slb->esid = (ea & ESID_MASK) | SLB_ESID_V;
> -
>  	switch (REGION_ID(ea)) {
>  	case USER_REGION_ID:
>  		pr_devel("%s: 0x%llx -- USER_REGION_ID\n", __func__, ea);
> @@ -133,6 +131,7 @@ int copro_calculate_slb(struct mm_struct *mm, u64 ea, struct copro_slb *slb)
>  	vsid |= mmu_psize_defs[psize].sllp |
>  		((ssize == MMU_SEGSIZE_1T) ? SLB_VSID_B_1T : 0);
>  
> +	slb->esid = (ea & (ssize == MMU_SEGSIZE_1T ? ESID_MASK_1T : ESID_MASK)) | SLB_ESID_V;
>  	slb->vsid = vsid;
>  
>  	return 0;
> diff --git a/drivers/misc/cxl/fault.c b/drivers/misc/cxl/fault.c
> index 69506eb..421cfd6 100644
> --- a/drivers/misc/cxl/fault.c
> +++ b/drivers/misc/cxl/fault.c
> @@ -21,60 +21,63 @@
>  
>  #include "cxl.h"
>  
> -static struct cxl_sste* find_free_sste(struct cxl_sste *primary_group,
> -				       bool sec_hash,
> -				       struct cxl_sste *secondary_group,
> -				       unsigned int *lru)
> +static bool sste_matches(struct cxl_sste *sste, struct copro_slb *slb)
>  {
> -	unsigned int i, entry;
> -	struct cxl_sste *sste, *group = primary_group;
> -
> -	for (i = 0; i < 2; i++) {
> -		for (entry = 0; entry < 8; entry++) {
> -			sste = group + entry;
> -			if (!(be64_to_cpu(sste->esid_data) & SLB_ESID_V))
> -				return sste;
> -		}
> -		if (!sec_hash)
> -			break;
> -		group = secondary_group;
> +	return ((sste->vsid_data == cpu_to_be64(slb->vsid)) &&
> +		(sste->esid_data == cpu_to_be64(slb->esid)));
> +}
> +
> +/* This finds a free SSTE and checks to see if it's already in table */
> +static struct cxl_sste* find_free_sste(struct cxl_context *ctx,
> +				       struct copro_slb *slb)

the name is confusing. If you want to keep the name, can you also
specify that it return NULL, if it finds a matching entry. IIUC that
is the real part of the fix for the problem mentioned ?

> +{
> +	struct cxl_sste *primary, *sste, *ret = NULL;
> +	unsigned int mask = (ctx->sst_size >> 7) - 1; /* SSTP0[SegTableSize] */
> +	unsigned int entry;
> +	unsigned int hash;
> +
> +	if (slb->vsid & SLB_VSID_B_1T)
> +		hash = (slb->esid >> SID_SHIFT_1T) & mask;
> +	else /* 256M */
> +		hash = (slb->esid >> SID_SHIFT) & mask;
> +
> +	primary = ctx->sstp + (hash << 3);
> +	sste = primary;
> +
> +	for (entry = 0; entry < 8; entry++) {
> +		if (!ret && !(be64_to_cpu(sste->esid_data) & SLB_ESID_V))
> +			ret = sste;
> +		if (sste_matches(sste, slb))
> +			return NULL;
> +		sste++;
>  	}
> +	if (ret)
> +		return ret;
> +
>  	/* Nothing free, select an entry to cast out */
> -	if (sec_hash && (*lru & 0x8))
> -		sste = secondary_group + (*lru & 0x7);
> -	else
> -		sste = primary_group + (*lru & 0x7);
> -	*lru = (*lru + 1) & 0xf;
> +	ret = primary + ctx->sst_lru;
> +	ctx->sst_lru = (ctx->sst_lru + 1) & 0x7;
>  
> -	return sste;
> +	return ret;
>  }
>  
>  static void cxl_load_segment(struct cxl_context *ctx, struct copro_slb *slb)
>  {
>  	/* mask is the group index, we search primary and secondary here. */
> -	unsigned int mask = (ctx->sst_size >> 7)-1; /* SSTP0[SegTableSize] */
> -	bool sec_hash = 1;
>  	struct cxl_sste *sste;
> -	unsigned int hash;
>  	unsigned long flags;
>  
> -
> -	sec_hash = !!(cxl_p1n_read(ctx->afu, CXL_PSL_SR_An) & CXL_PSL_SR_An_SC);
> -
> -	if (slb->vsid & SLB_VSID_B_1T)
> -		hash = (slb->esid >> SID_SHIFT_1T) & mask;
> -	else /* 256M */
> -		hash = (slb->esid >> SID_SHIFT) & mask;
> -
>  	spin_lock_irqsave(&ctx->sste_lock, flags);
> -	sste = find_free_sste(ctx->sstp + (hash << 3), sec_hash,
> -			      ctx->sstp + ((~hash & mask) << 3), &ctx->sst_lru);
> +	sste = find_free_sste(ctx, slb);
> +	if (!sste)
> +		goto out_unlock;
>  
>  	pr_devel("CXL Populating SST[%li]: %#llx %#llx\n",
>  			sste - ctx->sstp, slb->vsid, slb->esid);
>  
>  	sste->vsid_data = cpu_to_be64(slb->vsid);
>  	sste->esid_data = cpu_to_be64(slb->esid);
> +out_unlock:
>  	spin_unlock_irqrestore(&ctx->sste_lock, flags);
>  }
>  
> diff --git a/drivers/misc/cxl/native.c b/drivers/misc/cxl/native.c
> index 623286a..d47532e 100644
> --- a/drivers/misc/cxl/native.c
> +++ b/drivers/misc/cxl/native.c
> @@ -417,7 +417,7 @@ static int attach_afu_directed(struct cxl_context *ctx, u64 wed, u64 amr)
>  	ctx->elem->haurp = 0; /* disable */
>  	ctx->elem->sdr = cpu_to_be64(mfspr(SPRN_SDR1));
>  
> -	sr = CXL_PSL_SR_An_SC;
> +	sr = 0;
>  	if (ctx->master)
>  		sr |= CXL_PSL_SR_An_MP;
>  	if (mfspr(SPRN_LPCR) & LPCR_TC)
> @@ -508,7 +508,7 @@ static int attach_dedicated(struct cxl_context *ctx, u64 wed, u64 amr)
>  	u64 sr;
>  	int rc;
>  
> -	sr = CXL_PSL_SR_An_SC;
> +	sr = 0;

What is this change about ?

>  	set_endian(sr);
>  	if (ctx->master)
>  		sr |= CXL_PSL_SR_An_MP;
> -- 
> 2.1.1
>
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