[v3] Add AMD Secure Nested Paging (SEV-SNP) support

[PATCH v3 29/49] i386/sev: Don't disable block discarding for SNP

Posted by Michael Roth 1 year, 10 months ago

SEV/SEV-ES rely on pinned memory to back guest RAM so discarding
isn't actually possible. With SNP, only guest_memfd pages are used
for private guest memory, so discarding of shared memory is still
possible, so only disable discard for SEV/SEV-ES.

Signed-off-by: Michael Roth <michael.roth@amd.com>
---
 target/i386/sev.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/target/i386/sev.c b/target/i386/sev.c
index 134e8f7c22..43e6c0172f 100644
--- a/target/i386/sev.c
+++ b/target/i386/sev.c
@@ -888,10 +888,18 @@ static int sev_kvm_init(ConfidentialGuestSupport *cgs, Error **errp)
     uint32_t host_cbitpos;
     struct sev_user_data_status status = {};
 
-    ret = ram_block_discard_disable(true);
-    if (ret) {
-        error_report("%s: cannot disable RAM discard", __func__);
-        return -1;
+    /*
+     * SEV/SEV-ES rely on pinned memory to back guest RAM so discarding
+     * isn't actually possible. With SNP, only guest_memfd pages are used
+     * for private guest memory, so discarding of shared memory is still
+     * possible..
+     */
+    if (!sev_snp_enabled()) {
+        ret = ram_block_discard_disable(true);
+        if (ret) {
+            error_report("%s: cannot disable RAM discard", __func__);
+            return -1;
+        }
     }
 
     sev_common->state = SEV_STATE_UNINIT;
-- 
2.25.1

Re: [PATCH v3 29/49] i386/sev: Don't disable block discarding for SNP

Posted by Daniel P. Berrangé 1 year, 10 months ago

On Wed, Mar 20, 2024 at 03:39:25AM -0500, Michael Roth wrote:
> SEV/SEV-ES rely on pinned memory to back guest RAM so discarding
> isn't actually possible. With SNP, only guest_memfd pages are used
> for private guest memory, so discarding of shared memory is still
> possible, so only disable discard for SEV/SEV-ES.
> 
> Signed-off-by: Michael Roth <michael.roth@amd.com>
> ---
>  target/i386/sev.c | 16 ++++++++++++----
>  1 file changed, 12 insertions(+), 4 deletions(-)
> 
> diff --git a/target/i386/sev.c b/target/i386/sev.c
> index 134e8f7c22..43e6c0172f 100644
> --- a/target/i386/sev.c
> +++ b/target/i386/sev.c
> @@ -888,10 +888,18 @@ static int sev_kvm_init(ConfidentialGuestSupport *cgs, Error **errp)
>      uint32_t host_cbitpos;
>      struct sev_user_data_status status = {};
>  
> -    ret = ram_block_discard_disable(true);
> -    if (ret) {
> -        error_report("%s: cannot disable RAM discard", __func__);
> -        return -1;
> +    /*
> +     * SEV/SEV-ES rely on pinned memory to back guest RAM so discarding
> +     * isn't actually possible. With SNP, only guest_memfd pages are used
> +     * for private guest memory, so discarding of shared memory is still
> +     * possible..
> +     */
> +    if (!sev_snp_enabled()) {
> +        ret = ram_block_discard_disable(true);
> +        if (ret) {
> +            error_report("%s: cannot disable RAM discard", __func__);
> +            return -1;
> +        }
>      }

Pre-existing code bug, but this method must use 'error_setg' to fill
the 'Error **errp' parameter.


With regards,
Daniel
-- 
|: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org         -o-            https://fstop138.berrange.com :|
|: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|