Re: [PATCH v2] KVM: s390: gaccess: Refactor access address range check

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On 10/28/21 15:55, Janis Schoetterl-Glausch 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>

---
  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 0d11cea92603..7725dd7566ed 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

I'd add an empty line here.
Apart from that this is a very nice cleanup.

+ * @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 gpas 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 nr_pages, idx;
  	unsigned int fragment_len;
-	unsigned long pages_array[2];
-	unsigned long *pages;
+	unsigned long gpa_array[2];
+	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;





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