[PATCH] qemu.conf: add max-ram-below-4g option

[Daniel P. Berrangé]

On Wed, Apr 21, 2021 at 04:25:28PM +0800, yezhiyong wrote:
> Limit the amount of ram below 4G. This helps in scenarios like
> GPU passthrough when the GPA used by DMA device is conflict with
> the decode window of a host bridge and the address translation
> request to iommu isn't launched, which causes address overlapping.
> Note that currently this can be triggered by some abnormal behavior
> of hardware.
> 
> In the general case, this option needs to be configured when virtual
> machines share the same host, which is why using qemu.conf to support
> per-host configuration.

This memory layout setting affects the guest ABI, which in turn
means it is something that must be preserved upon live migration.
Using qemu.conf for this is dangerous because nothing will be
ensuring that guest ABI is perserved.

IOW, to support this feature definitely needs an attribute
in the main doain XML schema.

> 
> Signed-off-by: yezhiyong <yezhiyong at bytedance.com>
> Signed-off-by: zhenwei pi <pizhenwei at bytedance.com>
> Signed-off-by: zhangruien <zhangruien at bytedance.com>
> ---
>  src/qemu/qemu.conf      | 8 ++++++++
>  src/qemu/qemu_command.c | 4 ++++
>  src/qemu/qemu_conf.c    | 5 +++++
>  src/qemu/qemu_conf.h    | 1 +
>  4 files changed, 18 insertions(+)
> 
> diff --git a/src/qemu/qemu.conf b/src/qemu/qemu.conf
> index 8722dc169c..f09c89486e 100644
> --- a/src/qemu/qemu.conf
> +++ b/src/qemu/qemu.conf
> @@ -898,6 +898,14 @@
>  # NOTE: big files will be stored here
>  #memory_backing_dir = "/var/lib/libvirt/qemu/ram"
>  
> +# Limit the amount of ram below 4G. This helps in scenarios like
> +# GPU passthrough when the GPA used by DMA device is comflict with
> +# the decode window of a host bridge and the address translation
> +# request to iommu isn't launched, which causes address overlapping.
> +# Note that currently this can be triggered by some abnormal behavior
> +# of hardware.
> +#max_ram_below_4g = "2G"
> +
>  # Path to the SCSI persistent reservations helper. This helper is
>  # used whenever <reservations/> are enabled for SCSI LUN devices.
>  #pr_helper = "/usr/bin/qemu-pr-helper"
> diff --git a/src/qemu/qemu_command.c b/src/qemu/qemu_command.c
> index 2f69a79bc0..fd5e14c500 100644
> --- a/src/qemu/qemu_command.c
> +++ b/src/qemu/qemu_command.c
> @@ -6960,6 +6960,10 @@ qemuBuildMachineCommandLine(virCommand *cmd,
>                            cfg->dumpGuestCore ? "on" : "off");
>      }
>  
> +    if (cfg->maxRamBelow4G)
> +        virBufferAsprintf(&buf, ",max-ram-below-4g=%s",
> +                          cfg->maxRamBelow4G);
> +
>      if (def->mem.nosharepages)
>          virBufferAddLit(&buf, ",mem-merge=off");
>  
> diff --git a/src/qemu/qemu_conf.c b/src/qemu/qemu_conf.c
> index 916a3d36ee..b718995870 100644
> --- a/src/qemu/qemu_conf.c
> +++ b/src/qemu/qemu_conf.c
> @@ -384,6 +384,8 @@ static void virQEMUDriverConfigDispose(void *obj)
>      g_strfreev(cfg->capabilityfilters);
>  
>      g_free(cfg->deprecationBehavior);
> +
> +    g_free(cfg->maxRamBelow4G);
>  }
>  
>  
> @@ -1001,6 +1003,9 @@ virQEMUDriverConfigLoadMemoryEntry(virQEMUDriverConfig *cfg,
>      g_autofree char *dir = NULL;
>      int rc;
>  
> +    if (virConfGetValueString(conf, "max_ram_below_4g", &cfg->maxRamBelow4G) < 0)
> +        return -1;
> +
>      if ((rc = virConfGetValueString(conf, "memory_backing_dir", &dir)) < 0) {
>          return -1;
>      } else if (rc > 0) {
> diff --git a/src/qemu/qemu_conf.h b/src/qemu/qemu_conf.h
> index 2f64e39a18..ff558e2fdb 100644
> --- a/src/qemu/qemu_conf.h
> +++ b/src/qemu/qemu_conf.h
> @@ -216,6 +216,7 @@ struct _virQEMUDriverConfig {
>      bool virtiofsdDebug;
>  
>      char *memoryBackingDir;
> +    char *maxRamBelow4G;
>  
>      uid_t swtpm_user;
>      gid_t swtpm_group;
> -- 
> 2.24.3 (Apple Git-128)
> 

Regards,
Daniel
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