On 8/18/21 12:08 PM, Claudio Imbrenda wrote:

On Mon, 16 Aug 2021 17:07:17 +0200
Janis Schoetterl-Glausch [off-list ref] wrote:

quoted

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.

I like the idea, but see a few nits below

quoted

Signed-off-by: Janis Schoetterl-Glausch <redacted>
---
 arch/s390/kvm/gaccess.c | 91 ++++++++++++++++++-----------------------
 1 file changed, 39 insertions(+), 52 deletions(-)

diff --git a/arch/s390/kvm/gaccess.c b/arch/s390/kvm/gaccess.c
index df83de0843de..e5a19d8b30e2 100644
--- a/arch/s390/kvm/gaccess.c
+++ b/arch/s390/kvm/gaccess.c

@@ -794,35 +794,45 @@ 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)
+/* Stores the gpas for each page in a real/virtual range into @gpas
+ * Modifies the 'struct kvm_s390_pgm_info pgm' member of @vcpu in the same
+ * way read_guest/write_guest do, the meaning of the return value is likewise

this comment is a bit confusing; why telling us to look what a
different function is doing?

either don't mention this at all (since it's more or less the expected
behaviour), or explain in full what's going on

Yeah, it's not ideal. I haven't decided yet what I'll do.
I think a comment would be helpful, and it may be expected behavior only if one has
looked at the code for long enough :).

quoted

+ * the same.
+ * If @gpas is NULL only the checks are performed.
+ */
+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 long gpa;
+	unsigned int seg;
+	unsigned int offset = offset_in_page(ga);
 	int lap_enabled, rc = 0;
 	enum prot_type prot;
 
 	lap_enabled = low_address_protection_enabled(vcpu, asce);
-	while (nr_pages) {
+	while ((seg = min(PAGE_SIZE - offset, len)) != 0) {

I'm not terribly fond of assignments-as-values; moreover offset is used
only once.

why not something like:

	seg = min(PAGE_SIZE - offset, len);
	while (seg) {

		...

		seg = min(PAGE_SIZE, len);
	}

or maybe even:

	seg = min(PAGE_SIZE - offset, len);
	for (; seg; seg = min(PAGE_SIZE, len)) {

(although the one with the while is perhaps more readable)

That code pattern is not entirely uncommon, but I'll change it to:

	while(min(PAGE_SIZE - offset, len) > 0) {
		seg = min(PAGE_SIZE - offset, len);
		...
	}

which I think reads better than having the assignment at the end.
I assume the compiler gets rid of the redundancy.

[...]

quoted

@@ -845,10 +855,10 @@ 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 seg;
-	unsigned long pages_array[2];
-	unsigned long *pages;
+	unsigned long gpa_array[2];
+	unsigned long *gpas;

reverse Christmas tree?

also, since you're touching this: have you checked if a different size
for the array would bring any benefit?
2 seems a little too small, but I have no idea if anything bigger would
bring any advantages.

I have not checked it, no. When emulating instructions, you would only need >2
entries if an operand is >8k or >4k and weirdly aligned, hardly seems like a common occurrence.
On the other hand, bumping it up should not have any negative consequences.
I'll leave it as is.

[...]