renderer: transform feedback fixes

Impedance match the gallium and GL transform APIs,
we can't begin before draw vbo but we should only
end when the guest disconnects all buffers

using tfb pause/resume to make things better.

fixes: ext_transform_feedback-intervening-read

src/vrend_renderer.c

@@ -246,6 +246,11 @@ struct vrend_viewport {
    GLclampd near_val, far_val;
 };
 
+#define XFB_STATE_OFF 0
+#define XFB_STATE_STARTED_NEED_BEGIN 1
+#define XFB_STATE_STARTED 2
+#define XFB_STATE_PAUSED 3
+
 struct vrend_sub_context {
    struct list_head head;
 
@@ -327,6 +332,8 @@ struct vrend_sub_context {
 
    struct pipe_rasterizer_state hw_rs_state;
    struct pipe_blend_state hw_blend_state;
+
+   int xfb_state;
 };
 
 struct vrend_context {
@@ -2365,12 +2372,15 @@ void vrend_draw_vbo(struct vrend_context *ctx,
       glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0);
 
 //   vrend_ctx_restart_queries(ctx);
-
+   if (ctx->sub->xfb_state == XFB_STATE_STARTED_NEED_BEGIN) {
-   if (ctx->sub->num_so_targets) {
       if (ctx->sub->gs)
          glBeginTransformFeedback(get_gs_xfb_mode(ctx->sub->gs->sinfo.gs_out_prim));
       else
          glBeginTransformFeedback(get_xfb_mode(info->mode));
+      ctx->sub->xfb_state = XFB_STATE_STARTED;
+   } else if (ctx->sub->xfb_state == XFB_STATE_PAUSED) {
+      glResumeTransformFeedback();
+      ctx->sub->xfb_state = XFB_STATE_STARTED;
    }
 
    if (info->primitive_restart) {
@@ -2422,15 +2432,17 @@ void vrend_draw_vbo(struct vrend_context *ctx,
          glDrawElements(mode, info->count, elsz, (void *)(unsigned long)ctx->sub->ib.offset);
    }
 
-   if (ctx->sub->num_so_targets)
-      glEndTransformFeedback();
-
    if (info->primitive_restart) {
       if (vrend_state.have_nv_prim_restart)
          glDisableClientState(GL_PRIMITIVE_RESTART_NV);
       else if (vrend_state.have_gl_prim_restart)
          glDisable(GL_PRIMITIVE_RESTART);
    }
+
+   if (ctx->sub->xfb_state == XFB_STATE_STARTED) {
+     glPauseTransformFeedback();
+     ctx->sub->xfb_state = XFB_STATE_PAUSED;
+   }
 }
 
 static GLenum translate_blend_func(uint32_t pipe_blend)
@@ -4412,7 +4424,7 @@ static void vrend_hw_emit_streamout_targets(struct vrend_context *ctx)
    int i;
 
    for (i = 0; i < ctx->sub->num_so_targets; i++) {
-      if (ctx->sub->so_targets[i]->buffer_offset)
+      if (ctx->sub->so_targets[i]->buffer_offset || ctx->sub->so_targets[i]->buffer_size < ctx->sub->so_targets[i]->buffer->base.width0)
          glBindBufferRange(GL_TRANSFORM_FEEDBACK_BUFFER, i, ctx->sub->so_targets[i]->buffer->id, ctx->sub->so_targets[i]->buffer_offset, ctx->sub->so_targets[i]->buffer_size);
       else
          glBindBufferBase(GL_TRANSFORM_FEEDBACK_BUFFER, i, ctx->sub->so_targets[i]->buffer->id);
@@ -4429,6 +4441,12 @@ void vrend_set_streamout_targets(struct vrend_context *ctx,
    int old_num = ctx->sub->num_so_targets;
 
    ctx->sub->num_so_targets = num_targets;
+
+   if (num_targets == 0 && (ctx->sub->xfb_state == XFB_STATE_STARTED || ctx->sub->xfb_state == XFB_STATE_PAUSED)) {
+      glEndTransformFeedback();
+      ctx->sub->xfb_state = XFB_STATE_OFF;
+   }
+
    for (i = 0; i < num_targets; i++) {
       target = vrend_object_lookup(ctx->sub->object_hash, handles[i], VIRGL_OBJECT_STREAMOUT_TARGET);
       if (!target) {
@@ -4438,10 +4456,18 @@ void vrend_set_streamout_targets(struct vrend_context *ctx,
       vrend_so_target_reference(&ctx->sub->so_targets[i], target);
    }
 
-   for (i = num_targets; i < old_num; i++)
+   for (i = num_targets; i < old_num; i++) {
+      glBindBufferBase(GL_TRANSFORM_FEEDBACK_BUFFER, i, 0);
       vrend_so_target_reference(&ctx->sub->so_targets[i], NULL);
+   }
 
    vrend_hw_emit_streamout_targets(ctx);
+
+   if (ctx->sub->num_so_targets) {
+      if (ctx->sub->xfb_state == XFB_STATE_OFF)
+        ctx->sub->xfb_state = XFB_STATE_STARTED_NEED_BEGIN;
+   }
+
 }
 
 static void vrend_resource_buffer_copy(struct vrend_context *ctx,