Date:   Tue, 17 Oct 2017 23:40:30 +0200
From:   Christoffer Dall <cdall@...aro.org>
To:     Eric Auger <eric.auger@...hat.com>
Cc:     eric.auger.pro@...il.com, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, kvmarm@...ts.cs.columbia.edu,
        marc.zyngier@....com, peter.maydell@...aro.org,
        andre.przywara@....com, wanghaibin.wang@...wei.com,
        wu.wubin@...wei.com, drjones@...hat.com, wei@...hat.com
Subject: Re: [PATCH v4 01/11] KVM: arm/arm64: vgic-its: fix return value for
 device table restore

On Tue, Oct 17, 2017 at 09:09:59AM +0200, Eric Auger wrote:
> AT the moment if ITT only contains invalid entries,
> vgic_its_restore_itt returns 1 and this is considered as
> an an error in vgic_its_restore_dte.
> 
> Also in case the device table only contains invalid entries,
> the table restore fails and this is not correct.
> 
> This patch fully revisits the errror handling while fixing those
> 2 bugs.
> 
> - entry_fn_t now takes a valid output paraleter

                                        parameter

> - scan_its_table() now returns <= 0 values and output 2 booleans,
                                                 outputs
>   valid and last.
> - vgic_its_restore_itt() now returns <= 0 values.
> - vgic_its_restore_device_tables() also returns <= 0 values.
> 
> With that patch we are able to properly handle the case where
> all data are invalid but we still are able to detect the case
> where a next entry was referenced by some valid entry and
> never found.
> 
> Fixes: 57a9a117154c93 (KVM: arm64: vgic-its: Device table save/restore)
> Fixes: eff484e0298da5 (KVM: arm64: vgic-its: ITT save and restore)
> Signed-off-by: Eric Auger <eric.auger@...hat.com>
> Reported-by: wanghaibin <wanghaibin.wang@...wei.com>
> 
> ---
> 
> need to CC stable
> 
> v3 -> v4:
> - set *valid at beginning of handle_l1_dte
> 
> v2 -> v3:
> - add comments
> - added valid parameter
> - vgic_its_restore_itt don't return +1 anymore
> - reword the commit message
> 
> v1 -> v2:
> - if (ret > 0) ret = 0
> ---
>  virt/kvm/arm/vgic/vgic-its.c | 95 ++++++++++++++++++++++++++++++++------------
>  1 file changed, 70 insertions(+), 25 deletions(-)
> 
> diff --git a/virt/kvm/arm/vgic/vgic-its.c b/virt/kvm/arm/vgic/vgic-its.c
> index f51c1e1..eea14a1 100644
> --- a/virt/kvm/arm/vgic/vgic-its.c
> +++ b/virt/kvm/arm/vgic/vgic-its.c
> @@ -1772,16 +1772,20 @@ static u32 compute_next_eventid_offset(struct list_head *h, struct its_ite *ite)
>  
>  /**
>   * entry_fn_t - Callback called on a table entry restore path
> + *
>   * @its: its handle
>   * @id: id of the entry
>   * @entry: pointer to the entry
>   * @opaque: pointer to an opaque data
> + * @valid: indicates whether valid data is associated to this entry
> + * (the entry itself in case of linear table or entries in the next level,
> + * in case of hierachical tables)
>   *
>   * Return: < 0 on error, 0 if last element was identified, id offset to next
>   * element otherwise
>   */
>  typedef int (*entry_fn_t)(struct vgic_its *its, u32 id, void *entry,
> -			  void *opaque);
> +			  void *opaque, bool *valid);
>  
>  /**
>   * scan_its_table - Scan a contiguous table in guest RAM and applies a function
> @@ -1794,29 +1798,34 @@ typedef int (*entry_fn_t)(struct vgic_its *its, u32 id, void *entry,
>   * @start_id: the ID of the first entry in the table
>   * (non zero for 2d level tables)
>   * @fn: function to apply on each entry
> + * @opaque: opaque data passed to @fn
> + * @valid: indicates whether the table contains any valid data
> + * @last: returns whether the last valid entry was decoded
>   *
> - * Return: < 0 on error, 0 if last element was identified, 1 otherwise
> - * (the last element may not be found on second level tables)
> + * Return: < 0 on error, 0 on success
>   */
>  static int scan_its_table(struct vgic_its *its, gpa_t base, int size, int esz,
> -			  int start_id, entry_fn_t fn, void *opaque)
> +			  int start_id, entry_fn_t fn, void *opaque,
> +			  bool *valid, bool *last)
>  {
>  	void *entry = kzalloc(esz, GFP_KERNEL);
>  	struct kvm *kvm = its->dev->kvm;
>  	unsigned long len = size;
>  	int id = start_id;
>  	gpa_t gpa = base;
> +	int next_offset = 0;
>  	int ret;
>  
>  	while (len > 0) {
> -		int next_offset;
>  		size_t byte_offset;
> +		bool entry_valid;
>  
>  		ret = kvm_read_guest(kvm, gpa, entry, esz);
>  		if (ret)
>  			goto out;
>  
> -		next_offset = fn(its, id, entry, opaque);
> +		next_offset = fn(its, id, entry, opaque, &entry_valid);
> +		*valid |= entry_valid;
>  		if (next_offset <= 0) {
>  			ret = next_offset;
>  			goto out;
> @@ -1827,9 +1836,15 @@ static int scan_its_table(struct vgic_its *its, gpa_t base, int size, int esz,
>  		gpa += byte_offset;
>  		len -= byte_offset;
>  	}
> -	ret =  1;
> -
> +	/*
> +	 * the table lookup was completed without identifying the
> +	 * last valid entry (ie. next_offset > 0).
> +	 */

but you never set last to false?  If you require the caller to set the
variable to false, that should be documented, but it's weird.

> +	ret = 0;
>  out:
> +	if (!next_offset)
> +		*last = true;
> +

so if we scan the entire table to the end we won't set last?  Isn't that
a bit strange?

Also, if we can get id of the valid out parameter and instead handle
that within this function, I don't think you'll need the 'last' return
value in vgic_its_restore_device_tables, and you could make this:

	if (!next_offset && last)
		*last = true;
	else if (last)
		*last = false;

>  	kfree(entry);
>  	return ret;
>  }
> @@ -1854,12 +1869,14 @@ static int vgic_its_save_ite(struct vgic_its *its, struct its_device *dev,
>  
>  /**
>   * vgic_its_restore_ite - restore an interrupt translation entry
> + *
>   * @event_id: id used for indexing
>   * @ptr: pointer to the ITE entry
>   * @opaque: pointer to the its_device
> + * @valid: indicates whether the ite is valid
>   */
>  static int vgic_its_restore_ite(struct vgic_its *its, u32 event_id,
> -				void *ptr, void *opaque)
> +				void *ptr, void *opaque, bool *valid)
>  {
>  	struct its_device *dev = (struct its_device *)opaque;
>  	struct its_collection *collection;
> @@ -1879,7 +1896,9 @@ static int vgic_its_restore_ite(struct vgic_its *its, u32 event_id,
>  	coll_id = val & KVM_ITS_ITE_ICID_MASK;
>  	lpi_id = (val & KVM_ITS_ITE_PINTID_MASK) >> KVM_ITS_ITE_PINTID_SHIFT;
>  
> -	if (!lpi_id)
> +	*valid = !!lpi_id;
> +
> +	if (!*valid)
>  		return 1; /* invalid entry, no choice but to scan next entry */
>  
>  	if (lpi_id < VGIC_MIN_LPI)
> @@ -1940,6 +1959,14 @@ static int vgic_its_save_itt(struct vgic_its *its, struct its_device *device)
>  	return 0;
>  }
>  
> +/**
> + * vgic_its_restore_itt - restore the ITT of a device
> + *
> + * @its: its handle
> + * @dev: device handle
> + *
> + * Return 0 on success, < 0 on error
> + */
>  static int vgic_its_restore_itt(struct vgic_its *its, struct its_device *dev)
>  {
>  	const struct vgic_its_abi *abi = vgic_its_get_abi(its);
> @@ -1947,9 +1974,15 @@ static int vgic_its_restore_itt(struct vgic_its *its, struct its_device *dev)
>  	int ret;
>  	int ite_esz = abi->ite_esz;
>  	size_t max_size = BIT_ULL(dev->num_eventid_bits) * ite_esz;
> +	bool valid = false, last = false;
>  
>  	ret = scan_its_table(its, base, max_size, ite_esz, 0,
> -			     vgic_its_restore_ite, dev);
> +			     vgic_its_restore_ite, dev, &valid, &last);
> +
> +	if (!ret && valid && !last) {
> +		/* a next element was referenced but not found */
> +		return -EINVAL;

So this is if we ever found a valid entry, but somehow it didn't lead us
to the last entry, right?  Can't you handle that within the
scan_its_table?

As I understand it, scan_its_table is in one of two modes, either it's
linearly scanning, or it found a valid entry, and it's jumping from one
entry to the next, given the offsets.  If it's in the second mode, and
finds an invalid entry, it should return an error.

I think you can also get rid of the '*valid = false; return 1;' thing,
which looks a bit strange.

> +	}
>  
>  	return ret;
>  }
> @@ -1985,29 +2018,29 @@ static int vgic_its_save_dte(struct vgic_its *its, struct its_device *dev,
>   * @id: device id the DTE corresponds to
>   * @ptr: kernel VA where the 8 byte DTE is located
>   * @opaque: unused
> + * @valid: indicates whether the dte is valid
>   *
>   * Return: < 0 on error, 0 if the dte is the last one, id offset to the
>   * next dte otherwise
>   */
>  static int vgic_its_restore_dte(struct vgic_its *its, u32 id,
> -				void *ptr, void *opaque)
> +				void *ptr, void *opaque, bool *valid)
>  {
>  	struct its_device *dev;
>  	gpa_t itt_addr;
>  	u8 num_eventid_bits;
>  	u64 entry = *(u64 *)ptr;
> -	bool valid;
>  	u32 offset;
>  	int ret;
>  
>  	entry = le64_to_cpu(entry);
>  
> -	valid = entry >> KVM_ITS_DTE_VALID_SHIFT;
> +	*valid = entry >> KVM_ITS_DTE_VALID_SHIFT;
>  	num_eventid_bits = (entry & KVM_ITS_DTE_SIZE_MASK) + 1;
>  	itt_addr = ((entry & KVM_ITS_DTE_ITTADDR_MASK)
>  			>> KVM_ITS_DTE_ITTADDR_SHIFT) << 8;
>  
> -	if (!valid)
> +	if (!*valid)
>  		return 1;
>  
>  	/* dte entry is valid */
> @@ -2082,13 +2115,14 @@ static int vgic_its_save_device_tables(struct vgic_its *its)
>   * @id: index of the entry in the L1 table
>   * @addr: kernel VA
>   * @opaque: unused
> + * @valid: indicates whether any dte entry was found
>   *
>   * L1 table entries are scanned by steps of 1 entry
>   * Return < 0 if error, 0 if last dte was found when scanning the L2
>   * table, +1 otherwise (meaning next L1 entry must be scanned)
>   */
>  static int handle_l1_dte(struct vgic_its *its, u32 id, void *addr,
> -			 void *opaque)
> +			 void *opaque, bool *valid)
>  {
>  	const struct vgic_its_abi *abi = vgic_its_get_abi(its);
>  	int l2_start_id = id * (SZ_64K / abi->dte_esz);
> @@ -2096,21 +2130,29 @@ static int handle_l1_dte(struct vgic_its *its, u32 id, void *addr,
>  	int dte_esz = abi->dte_esz;
>  	gpa_t gpa;
>  	int ret;
> +	bool last;
>  
>  	entry = le64_to_cpu(entry);
>  
> -	if (!(entry & KVM_ITS_L1E_VALID_MASK))
> +	*valid = entry & KVM_ITS_L1E_VALID_MASK;
> +
> +	if (!*valid)
>  		return 1;
>  
>  	gpa = entry & KVM_ITS_L1E_ADDR_MASK;
>  
>  	ret = scan_its_table(its, gpa, SZ_64K, dte_esz,
> -			     l2_start_id, vgic_its_restore_dte, NULL);
> +			     l2_start_id, vgic_its_restore_dte, NULL,
> +			     valid, &last);
>  
> -	if (ret <= 0)
> -		return ret;
> +	/*
> +	 * if the last dte has not been found in this L2 table, we
> +	 * need to scan the next L1 entry
> +	 */
> +	if (!ret && !last)
> +		return 1;
>  
> -	return 1;
> +	return ret;
>  }
>  
>  /**
> @@ -2124,6 +2166,7 @@ static int vgic_its_restore_device_tables(struct vgic_its *its)
>  	int l1_esz, ret;
>  	int l1_tbl_size = GITS_BASER_NR_PAGES(baser) * SZ_64K;
>  	gpa_t l1_gpa;
> +	bool valid = false, last = false;
>  
>  	if (!(baser & GITS_BASER_VALID))
>  		return 0;
> @@ -2133,15 +2176,17 @@ static int vgic_its_restore_device_tables(struct vgic_its *its)
>  	if (baser & GITS_BASER_INDIRECT) {
>  		l1_esz = GITS_LVL1_ENTRY_SIZE;
>  		ret = scan_its_table(its, l1_gpa, l1_tbl_size, l1_esz, 0,
> -				     handle_l1_dte, NULL);
> +				     handle_l1_dte, NULL, &valid, &last);
>  	} else {
>  		l1_esz = abi->dte_esz;
>  		ret = scan_its_table(its, l1_gpa, l1_tbl_size, l1_esz, 0,
> -				     vgic_its_restore_dte, NULL);
> +				     vgic_its_restore_dte, NULL, &valid, &last);
>  	}
>  
> -	if (ret > 0)
> -		ret = -EINVAL;
> +	if (!ret && valid && !last) {
> +		/* a next element was referenced but not found */
> +		return -EINVAL;
> +	}

Same comment as above.

>  
>  	return ret;
>  }
> -- 
> 2.5.5
> 
Thanks,
-Christoffer

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