On Mon, 2021-11-15 at 16:50 +0000, David Woodhouse wrote:
> + /*
> + * Now we have a valid (protected by srcu) userspace HVA in
> + * ghc->hva which points to the struct vcpu_info. If there
> + * are any bits in the in-kernel evtchn_pending_sel then
> + * we need to write those to the guest vcpu_info and set
> + * its evtchn_upcall_pending flag. If there aren't any bits
> + * to add, we only want to *check* evtchn_upcall_pending.
> + */
> + if (evtchn_pending_sel) {
> + bool long_mode = v->kvm->arch.xen.long_mode;
> +
> + if (!user_access_begin((void *)ghc->hva, sizeof(struct vcpu_info)))
> + return 0;
> +
> + if (IS_ENABLED(CONFIG_64BIT) && long_mode) {
> + struct vcpu_info __user *vi = (void *)ghc->hva;
> +
> + /* Attempt to set the evtchn_pending_sel bits in the
> + * guest, and if that succeeds then clear the same
> + * bits in the in-kernel version. */
> + asm volatile("1:\t" LOCK_PREFIX "orq %0, %1\n"
> + "\tnotq %0\n"
> + "\t" LOCK_PREFIX "andq %0, %2\n"
> + "2:\n"
> + "\t.section .fixup,\"ax\"\n"
> + "3:\tjmp\t2b\n"
> + "\t.previous\n"
> + _ASM_EXTABLE_UA(1b, 3b)
> + : "=r" (evtchn_pending_sel)
> + : "m" (vi->evtchn_pending_sel),
> + "m" (v->arch.xen.evtchn_pending_sel),
> + "0" (evtchn_pending_sel));
> + } else {
> + struct compat_vcpu_info __user *vi = (void *)ghc->hva;
> + u32 evtchn_pending_sel32 = evtchn_pending_sel;
> +
> + /* Attempt to set the evtchn_pending_sel bits in the
> + * guest, and if that succeeds then clear the same
> + * bits in the in-kernel version. */
> + asm volatile("1:\t" LOCK_PREFIX "orl %0, %1\n"
> + "\tnotl %0\n"
> + "\t" LOCK_PREFIX "andl %0, %2\n"
> + "2:\n"
> + "\t.section .fixup,\"ax\"\n"
> + "3:\tjmp\t2b\n"
> + "\t.previous\n"
> + _ASM_EXTABLE_UA(1b, 3b)
> + : "=r" (evtchn_pending_sel32)
> + : "m" (vi->evtchn_pending_sel),
> + "m" (v->arch.xen.evtchn_pending_sel),
> + "0" (evtchn_pending_sel32));
> + }
> + rc = 1;
> + unsafe_put_user(rc, (u8 __user *)ghc->hva + offset, err);
> +
> + err:
> + user_access_end();
> +
> + mark_page_dirty_in_slot(v->kvm, ghc->memslot, ghc->gpa >> PAGE_SHIFT);
> + } else {
> + __get_user(rc, (u8 __user *)ghc->hva + offset);
> + }
I will probably concede that my assertion that "it already has a
perfectly serviceable userspace HVA and it's just a matter of writing a
trivial bit of inline asm" is probably stretching the definition of the
word "trivial" a little bit.
I can convert this bit to use a gfn_to_gpa_cache for the vcpu_info too,
once the dust settles on the implementation of that.
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