builtin-run: Rework RAM size validation

host_ram_size() uses sysconf() to calculate the available ram, and
sysconf() can fail. When that happens, host_ram_size() returns 0. kvmtool
warns the user when the configured VM ram size exceeds the size of the
host's memory, but doesn't take into account that host_ram_size() can
return 0. If the function returns zero, skip the warning.

Since this can only happen when the user sets the memory size (via the
-m/--mem command line argument), skip the check entirely if the user hasn't
set it. Move the check to kvm_run_validate_cfg(), as it checks for valid
user configuration.

Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Reviewed-and-Tested-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/20220616134828.129006-4-alexandru.elisei@arm.com
Signed-off-by: Will Deacon <will@kernel.org>

1 files changed, 13 insertions, 7 deletions

diff --git a/builtin-run.c b/builtin-run.c
index 2bf93fe1..e1770b3c 100644
--- a/builtin-run.c
+++ b/builtin-run.c
@@ -509,6 +509,8 @@ static void kvm_run_set_real_cmdline(struct kvm *kvm)
 
 static void kvm_run_validate_cfg(struct kvm *kvm)
 {
+	u64 available_ram;
+
 	if (kvm->cfg.kernel_filename && kvm->cfg.firmware_filename)
 		die("Only one of --kernel or --firmware can be specified");
 
@@ -518,6 +520,17 @@ static void kvm_run_validate_cfg(struct kvm *kvm)
 
 	if (kvm->cfg.firmware_filename && kvm->cfg.initrd_filename)
 		pr_warning("Ignoring initrd file when loading a firmware image");
+
+	if (kvm->cfg.ram_size) {
+		/* User specifies RAM size in megabytes. */
+		kvm->cfg.ram_size <<= MB_SHIFT;
+		available_ram = host_ram_size();
+		if (available_ram && kvm->cfg.ram_size > available_ram) {
+			pr_warning("Guest memory size %lluMB exceeds host physical RAM size %lluMB",
+				(unsigned long long)kvm->cfg.ram_size >> MB_SHIFT,
+				(unsigned long long)available_ram >> MB_SHIFT);
+		}
+	}
 }
 
 static struct kvm *kvm_cmd_run_init(int argc, const char **argv)
@@ -596,13 +609,6 @@ static struct kvm *kvm_cmd_run_init(int argc, const char **argv)
 
 	if (!kvm->cfg.ram_size)
 		kvm->cfg.ram_size = get_ram_size(kvm->cfg.nrcpus);
-	else
-		kvm->cfg.ram_size <<= MB_SHIFT;
-
-	if (kvm->cfg.ram_size > host_ram_size())
-		pr_warning("Guest memory size %lluMB exceeds host physical RAM size %lluMB",
-			(unsigned long long)kvm->cfg.ram_size >> MB_SHIFT,
-			(unsigned long long)host_ram_size() >> MB_SHIFT);
 
 	if (!kvm->cfg.dev)
 		kvm->cfg.dev = DEFAULT_KVM_DEV;