On Wed, Mar 27, 2019 at 03:21:02PM +0000, Julien Thierry wrote:

On 27/03/2019 10:33, Dave Martin wrote:

quoted

On Wed, Mar 27, 2019 at 09:47:42AM +0000, Julien Thierry wrote:

quoted

Hi Dave,

On 19/03/2019 17:52, Dave Martin wrote:

[...]

quoted

quoted

quoted

+static unsigned long num_sve_regs(const struct kvm_vcpu *vcpu)
+{
+	/* Only the first slice ever exists, for now */
+	const unsigned int slices = 1;

Nit: Might be worth introducing a macro/inline function for the number
of slices supported. This way, the day we need to change that, we only
need to look for that identifier.

... Reasonable point, but I wanted to avoid inventing anything
prematurely, partly because sve_reg_to_region() will need work in order
to support multiple slices (though it's not rocket science).

I could introduce something like the following:

static unsigned int sve_num_slices(const struct kvm_vcpu *vcpu)
{
	unsigned int slice_size = KVM_REG_SIZE(KVM_REG_ARM64_SVE_ZREG(0, 0));
	unsigned int slices = DIV_ROUND_UP(vcpu->arch.sve_max_vl, slice_size);

	/*
	 * For now, the SVE register ioctl access code won't work
	 * properly with multiple register slices.  KVM should prevent
	 * configuration of a vcpu with a maximum vector length large
	 * enough to trigger this:
	 */
	if (WARN_ON_ONCE(slices > 1))
		return 1;

	return slices;
}

This may be clearer, but felt a bit like overkill...

Thoughts?

Seems a bit overkill yes... I was more thinking of a define and the
person in charge of adding the slice support would just need to look for
references to that define to know (some of) the places that would need
rework/review.

So, unless someone else thinks it's good to introduce it right now you
can ignore that.

OK, how about the following?  This keeps things minimal, but should help
future maintainers know that something may need updating here in the
future. 

Cheers
---Dave

diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
index ea5219d..086ab05 100644
--- a/arch/arm64/kvm/guest.c
+++ b/arch/arm64/kvm/guest.c

@@ -289,6 +289,13 @@ static int set_sve_vls(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg)
 #define KVM_SVE_ZREG_SIZE KVM_REG_SIZE(KVM_REG_ARM64_SVE_ZREG(0, 0))
 #define KVM_SVE_PREG_SIZE KVM_REG_SIZE(KVM_REG_ARM64_SVE_PREG(0, 0))
 
+/*
+ * number of register slices required to cover each whole SVE register on vcpu
+ * NOTE: If you are tempted to modify this, you must also rework
+ * sve_reg_to_region() to match:
+ */
+#define vcpu_sve_slices(vcpu) 1
+
 /* Bounds of a single SVE register slice within vcpu->arch.sve_state */
 struct sve_state_reg_region {
 	unsigned int koffset;	/* offset into sve_state in kernel memory */

@@ -505,7 +512,7 @@ static int get_timer_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg)
 static unsigned long num_sve_regs(const struct kvm_vcpu *vcpu)
 {
 	/* Only the first slice ever exists, for now */
-	const unsigned int slices = 1;
+	const unsigned int slices = vcpu_sve_slices(vcpu);
 
 	if (!vcpu_has_sve(vcpu))
 		return 0;

@@ -521,7 +528,7 @@ static int copy_sve_reg_indices(const struct kvm_vcpu *vcpu,
 				u64 __user *uindices)
 {
 	/* Only the first slice ever exists, for now */
-	const unsigned int slices = 1;
+	const unsigned int slices = vcpu_sve_slices(vcpu);
 	u64 reg;
 	unsigned int i, n;
 	int num_regs = 0;

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