Re: [PATCH v2 6/6] KVM: SEV: use scoped mutex guard in sev_asid_new()

[Sean Christopherson <seanjc@google.com>]

On Tue, Jan 20, 2026, Carlos López wrote:
> Simplify the lock management in sev_asid_new() by using a mutex guard,
> automatically releasing the mutex when following the goto.
> 
> Signed-off-by: Carlos López <clopez@suse.de>
> ---
>  arch/x86/kvm/svm/sev.c | 24 ++++++++++--------------
>  1 file changed, 10 insertions(+), 14 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index d3fa0963465d..d8d5c3a703f9 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -231,24 +231,20 @@ static int sev_asid_new(struct kvm_sev_info *sev, unsigned long vm_type)
>  		return ret;
>  	}
>  
> -	mutex_lock(&sev_bitmap_lock);
> -
> +	scoped_guard(mutex, &sev_bitmap_lock) {
>  again:
> -	asid = find_next_zero_bit(sev_asid_bitmap, max_asid + 1, min_asid);
> -	if (asid > max_asid) {
> -		if (retry && __sev_recycle_asids(min_asid, max_asid)) {
> -			retry = false;
> -			goto again;
> +		asid = find_next_zero_bit(sev_asid_bitmap, max_asid + 1, min_asid);
> +		if (asid > max_asid) {
> +			if (retry && __sev_recycle_asids(min_asid, max_asid)) {
> +				retry = false;
> +				goto again;
> +			}
> +			ret = -EBUSY;
> +			goto e_uncharge;
>  		}
> -		mutex_unlock(&sev_bitmap_lock);
> -		ret = -EBUSY;
> -		goto e_uncharge;
> +		__set_bit(asid, sev_asid_bitmap);
>  	}

I think I'd prefer to throw this into a helper to avoid the goto within the
scoped guard.  FWIW, I also tried (quite hard) to replace the goto with a loop,
and couldn't come up with anything better.

No need for you to send a v3, I'll incorporate these patches into a larger series
(there are some locking goofs that need to be fixed, and at least one of these
patches will generate an annoying-but-easy-to-resolve conflict).

Thanks!

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index af84357dc954..249384e30320 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -198,6 +198,28 @@ static void sev_misc_cg_uncharge(struct kvm_sev_info *sev)
        misc_cg_uncharge(type, sev->misc_cg, 1);
 }
 
+static unsigned int sev_alloc_asid(unsigned int min_asid, unsigned int max_asid)
+{
+       unsigned int asid;
+       bool retry = true;
+
+       guard(mutex)(&sev_bitmap_lock);
+
+again:
+       asid = find_next_zero_bit(sev_asid_bitmap, max_asid + 1, min_asid);
+       if (asid > max_asid) {
+               if (retry && __sev_recycle_asids(min_asid, max_asid)) {
+                       retry = false;
+                       goto again;
+               }
+
+               return asid;
+       }
+
+       __set_bit(asid, sev_asid_bitmap);
+       return asid;
+}
+
 static int sev_asid_new(struct kvm_sev_info *sev, unsigned long vm_type)
 {
        /*
@@ -205,7 +227,6 @@ static int sev_asid_new(struct kvm_sev_info *sev, unsigned long vm_type)
         * SEV-ES-enabled guest can use from 1 to min_sev_asid - 1.
         */
        unsigned int min_asid, max_asid, asid;
-       bool retry = true;
        int ret;
 
        if (vm_type == KVM_X86_SNP_VM) {
@@ -238,24 +259,12 @@ static int sev_asid_new(struct kvm_sev_info *sev, unsigned long vm_type)
                return ret;
        }
 
-       mutex_lock(&sev_bitmap_lock);
-
-again:
-       asid = find_next_zero_bit(sev_asid_bitmap, max_asid + 1, min_asid);
+       asid = sev_alloc_asid(min_asid, max_asid);
        if (asid > max_asid) {
-               if (retry && __sev_recycle_asids(min_asid, max_asid)) {
-                       retry = false;
-                       goto again;
-               }
-               mutex_unlock(&sev_bitmap_lock);
                ret = -EBUSY;
                goto e_uncharge;
        }
 
-       __set_bit(asid, sev_asid_bitmap);
-
-       mutex_unlock(&sev_bitmap_lock);
-
        sev->asid = asid;
        return 0;
 e_uncharge: