Re: [PATCH v3 2/3] KVM: s390: gaccess: Refactor access address range check

[Claudio Imbrenda]

On Fri, 26 Nov 2021 17:45:48 +0100
Janis Schoetterl-Glausch <scgl@xxxxxxxxxxxxx> wrote:

> Do not round down the first address to the page boundary, just translate
> it normally, which gives the value we care about in the first place.
> Given this, translating a single address is just the special case of
> translating a range spanning a single page.
> 
> Make the output optional, so the function can be used to just check a
> range.
> 
> Signed-off-by: Janis Schoetterl-Glausch <scgl@xxxxxxxxxxxxx>
> Reviewed-by: Janosch Frank <frankja@xxxxxxxxxxxxx>

Reviewed-by: Claudio Imbrenda <imbrenda@xxxxxxxxxxxxx>

> ---
>  arch/s390/kvm/gaccess.c | 122 +++++++++++++++++++++++-----------------
>  1 file changed, 69 insertions(+), 53 deletions(-)
> 
> diff --git a/arch/s390/kvm/gaccess.c b/arch/s390/kvm/gaccess.c
> index 45966fbba182..c09659609d68 100644
> --- a/arch/s390/kvm/gaccess.c
> +++ b/arch/s390/kvm/gaccess.c
> @@ -794,35 +794,74 @@ static int low_address_protection_enabled(struct kvm_vcpu *vcpu,
>  	return 1;
>  }
>  
> -static int guest_page_range(struct kvm_vcpu *vcpu, unsigned long ga, u8 ar,
> -			    unsigned long *pages, unsigned long nr_pages,
> -			    const union asce asce, enum gacc_mode mode)
> +/**
> + * guest_range_to_gpas() - Calculate guest physical addresses of page fragments
> + * covering a logical range
> + * @vcpu: virtual cpu
> + * @ga: guest address, start of range
> + * @ar: access register
> + * @gpas: output argument, may be NULL
> + * @len: length of range in bytes
> + * @asce: address-space-control element to use for translation
> + * @mode: access mode
> + *
> + * Translate a logical range to a series of guest absolute addresses,
> + * such that the concatenation of page fragments starting at each gpa make up
> + * the whole range.
> + * The translation is performed as if done by the cpu for the given @asce, @ar,
> + * @mode and state of the @vcpu.
> + * If the translation causes an exception, its program interruption code is
> + * returned and the &struct kvm_s390_pgm_info pgm member of @vcpu is modified
> + * such that a subsequent call to kvm_s390_inject_prog_vcpu() will inject
> + * a correct exception into the guest.
> + * The resulting gpas are stored into @gpas, unless it is NULL.
> + *
> + * Note: All fragments except the first one start at the beginning of a page.
> + *       When deriving the boundaries of a fragment from a gpa, all but the last
> + *       fragment end at the end of the page.
> + *
> + * Return:
> + * * 0		- success
> + * * <0		- translation could not be performed, for example if  guest
> + *		  memory could not be accessed
> + * * >0		- an access exception occurred. In this case the returned value
> + *		  is the program interruption code and the contents of pgm may
> + *		  be used to inject an exception into the guest.
> + */
> +static int guest_range_to_gpas(struct kvm_vcpu *vcpu, unsigned long ga, u8 ar,
> +			       unsigned long *gpas, unsigned long len,
> +			       const union asce asce, enum gacc_mode mode)
>  {
>  	psw_t *psw = &vcpu->arch.sie_block->gpsw;
> +	unsigned int offset = offset_in_page(ga);
> +	unsigned int fragment_len;
>  	int lap_enabled, rc = 0;
>  	enum prot_type prot;
> +	unsigned long gpa;
>  
>  	lap_enabled = low_address_protection_enabled(vcpu, asce);
> -	while (nr_pages) {
> +	while (min(PAGE_SIZE - offset, len) > 0) {
> +		fragment_len = min(PAGE_SIZE - offset, len);
>  		ga = kvm_s390_logical_to_effective(vcpu, ga);
>  		if (mode == GACC_STORE && lap_enabled && is_low_address(ga))
>  			return trans_exc(vcpu, PGM_PROTECTION, ga, ar, mode,
>  					 PROT_TYPE_LA);
> -		ga &= PAGE_MASK;
>  		if (psw_bits(*psw).dat) {
> -			rc = guest_translate(vcpu, ga, pages, asce, mode, &prot);
> +			rc = guest_translate(vcpu, ga, &gpa, asce, mode, &prot);
>  			if (rc < 0)
>  				return rc;
>  		} else {
> -			*pages = kvm_s390_real_to_abs(vcpu, ga);
> -			if (kvm_is_error_gpa(vcpu->kvm, *pages))
> +			gpa = kvm_s390_real_to_abs(vcpu, ga);
> +			if (kvm_is_error_gpa(vcpu->kvm, gpa))
>  				rc = PGM_ADDRESSING;
>  		}
>  		if (rc)
>  			return trans_exc(vcpu, rc, ga, ar, mode, prot);
> -		ga += PAGE_SIZE;
> -		pages++;
> -		nr_pages--;
> +		if (gpas)
> +			*gpas++ = gpa;
> +		offset = 0;
> +		ga += fragment_len;
> +		len -= fragment_len;
>  	}
>  	return 0;
>  }
> @@ -831,10 +870,10 @@ int access_guest(struct kvm_vcpu *vcpu, unsigned long ga, u8 ar, void *data,
>  		 unsigned long len, enum gacc_mode mode)
>  {
>  	psw_t *psw = &vcpu->arch.sie_block->gpsw;
> -	unsigned long nr_pages, gpa, idx;
> -	unsigned long pages_array[2];
> +	unsigned long nr_pages, idx;
> +	unsigned long gpa_array[2];
>  	unsigned int fragment_len;
> -	unsigned long *pages;
> +	unsigned long *gpas;
>  	int need_ipte_lock;
>  	union asce asce;
>  	int rc;
> @@ -846,30 +885,28 @@ int access_guest(struct kvm_vcpu *vcpu, unsigned long ga, u8 ar, void *data,
>  	if (rc)
>  		return rc;
>  	nr_pages = (((ga & ~PAGE_MASK) + len - 1) >> PAGE_SHIFT) + 1;
> -	pages = pages_array;
> -	if (nr_pages > ARRAY_SIZE(pages_array))
> -		pages = vmalloc(array_size(nr_pages, sizeof(unsigned long)));
> -	if (!pages)
> +	gpas = gpa_array;
> +	if (nr_pages > ARRAY_SIZE(gpa_array))
> +		gpas = vmalloc(array_size(nr_pages, sizeof(unsigned long)));
> +	if (!gpas)
>  		return -ENOMEM;
>  	need_ipte_lock = psw_bits(*psw).dat && !asce.r;
>  	if (need_ipte_lock)
>  		ipte_lock(vcpu);
> -	rc = guest_page_range(vcpu, ga, ar, pages, nr_pages, asce, mode);
> +	rc = guest_range_to_gpas(vcpu, ga, ar, gpas, len, asce, mode);
>  	for (idx = 0; idx < nr_pages && !rc; idx++) {
> -		gpa = pages[idx] + offset_in_page(ga);
> -		fragment_len = min(PAGE_SIZE - offset_in_page(gpa), len);
> +		fragment_len = min(PAGE_SIZE - offset_in_page(gpas[idx]), len);
>  		if (mode == GACC_STORE)
> -			rc = kvm_write_guest(vcpu->kvm, gpa, data, fragment_len);
> +			rc = kvm_write_guest(vcpu->kvm, gpas[idx], data, fragment_len);
>  		else
> -			rc = kvm_read_guest(vcpu->kvm, gpa, data, fragment_len);
> +			rc = kvm_read_guest(vcpu->kvm, gpas[idx], data, fragment_len);
>  		len -= fragment_len;
> -		ga += fragment_len;
>  		data += fragment_len;
>  	}
>  	if (need_ipte_lock)
>  		ipte_unlock(vcpu);
> -	if (nr_pages > ARRAY_SIZE(pages_array))
> -		vfree(pages);
> +	if (nr_pages > ARRAY_SIZE(gpa_array))
> +		vfree(gpas);
>  	return rc;
>  }
>  
> @@ -911,8 +948,6 @@ int access_guest_real(struct kvm_vcpu *vcpu, unsigned long gra,
>  int guest_translate_address(struct kvm_vcpu *vcpu, unsigned long gva, u8 ar,
>  			    unsigned long *gpa, enum gacc_mode mode)
>  {
> -	psw_t *psw = &vcpu->arch.sie_block->gpsw;
> -	enum prot_type prot;
>  	union asce asce;
>  	int rc;
>  
> @@ -920,23 +955,7 @@ int guest_translate_address(struct kvm_vcpu *vcpu, unsigned long gva, u8 ar,
>  	rc = get_vcpu_asce(vcpu, &asce, gva, ar, mode);
>  	if (rc)
>  		return rc;
> -	if (is_low_address(gva) && low_address_protection_enabled(vcpu, asce)) {
> -		if (mode == GACC_STORE)
> -			return trans_exc(vcpu, PGM_PROTECTION, gva, 0,
> -					 mode, PROT_TYPE_LA);
> -	}
> -
> -	if (psw_bits(*psw).dat && !asce.r) {	/* Use DAT? */
> -		rc = guest_translate(vcpu, gva, gpa, asce, mode, &prot);
> -		if (rc > 0)
> -			return trans_exc(vcpu, rc, gva, 0, mode, prot);
> -	} else {
> -		*gpa = kvm_s390_real_to_abs(vcpu, gva);
> -		if (kvm_is_error_gpa(vcpu->kvm, *gpa))
> -			return trans_exc(vcpu, rc, gva, PGM_ADDRESSING, mode, 0);
> -	}
> -
> -	return rc;
> +	return guest_range_to_gpas(vcpu, gva, ar, gpa, 1, asce, mode);
>  }
>  
>  /**
> @@ -950,17 +969,14 @@ int guest_translate_address(struct kvm_vcpu *vcpu, unsigned long gva, u8 ar,
>  int check_gva_range(struct kvm_vcpu *vcpu, unsigned long gva, u8 ar,
>  		    unsigned long length, enum gacc_mode mode)
>  {
> -	unsigned long gpa;
> -	unsigned long currlen;
> +	union asce asce;
>  	int rc = 0;
>  
> +	rc = get_vcpu_asce(vcpu, &asce, gva, ar, mode);
> +	if (rc)
> +		return rc;
>  	ipte_lock(vcpu);
> -	while (length > 0 && !rc) {
> -		currlen = min(length, PAGE_SIZE - (gva % PAGE_SIZE));
> -		rc = guest_translate_address(vcpu, gva, ar, &gpa, mode);
> -		gva += currlen;
> -		length -= currlen;
> -	}
> +	rc = guest_range_to_gpas(vcpu, gva, ar, NULL, length, asce, mode);
>  	ipte_unlock(vcpu);
>  
>  	return rc;