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

[Daniel P . Berrangé]

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
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