KVM: selftests: Add string formatting options to ucall

Add more flexibility to guest debugging and testing by adding
GUEST_PRINTF() and GUEST_ASSERT_FMT() to the ucall framework.

Add a sized buffer to the ucall structure to hold the formatted string,
i.e. to allow the guest to easily resolve the string, and thus avoid the
ugly pattern of the host side having to make assumptions about the desired
format, as well as having to pass around a large number of parameters.

The buffer size was chosen to accommodate most use cases, and based on
similar usage.  E.g. printf() uses the same size buffer in
arch/x86/boot/printf.c.  And 1KiB ought to be enough for anybody.

Signed-off-by: Aaron Lewis <aaronlewis@google.com>
[sean: massage changelog, wrap macro param in ()]
Link: https://lore.kernel.org/r/20230729003643.1053367-8-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

1 files changed, 17 insertions, 0 deletions

diff --git a/tools/testing/selftests/kvm/lib/ucall_common.c b/tools/testing/selftests/kvm/lib/ucall_common.c
index 77ada362273d..b507db91139b 100644
--- a/tools/testing/selftests/kvm/lib/ucall_common.c
+++ b/tools/testing/selftests/kvm/lib/ucall_common.c
@@ -75,6 +75,23 @@ static void ucall_free(struct ucall *uc)
 	clear_bit(uc - ucall_pool->ucalls, ucall_pool->in_use);
 }
 
+void ucall_fmt(uint64_t cmd, const char *fmt, ...)
+{
+	struct ucall *uc;
+	va_list va;
+
+	uc = ucall_alloc();
+	uc->cmd = cmd;
+
+	va_start(va, fmt);
+	guest_vsnprintf(uc->buffer, UCALL_BUFFER_LEN, fmt, va);
+	va_end(va);
+
+	ucall_arch_do_ucall((vm_vaddr_t)uc->hva);
+
+	ucall_free(uc);
+}
+
 void ucall(uint64_t cmd, int nargs, ...)
 {
 	struct ucall *uc;