Fix CUDA FlashMLA-3 with quantized KV cache (#400)

Co-authored-by: Iwan Kawrakow <[email protected]>

Author: ikawrakow

ggml/src/ggml-cuda/fattn-new-mma.cu

@@ -1362,26 +1362,46 @@ void launch_fattn_new_mma(
         to_fp16(K_data, K_f16.ptr, 1, ggml_nelements(K), main_stream);
         K_data = (char *) K_f16.ptr;
 
-        const size_t bs = ggml_blck_size(K->type);
-        const size_t ts = ggml_type_size(K->type);
-
-        nb11 = nb11*bs*sizeof(half)/ts;
-        nb12 = nb12*bs*sizeof(half)/ts;
-        nb13 = nb13*bs*sizeof(half)/ts;
+        nb11 = K->ne[0]*sizeof(half);
+        nb12 = nb11*K->ne[1];
+        nb13 = nb12*K->ne[2];
+
+        // Original PR in llama.cpp. I don't think that can work when K is not contiguous (e.g., nb11 > nb12), there are
+        //                           gaps between the rows, etc., as ggml_get_to_fp16_cuda stores into contiguous memory.
+        //const size_t bs = ggml_blck_size(K->type);
+        //const size_t ts = ggml_type_size(K->type);
+
+        //nb11 = nb11*bs*sizeof(half)/ts;
+        //nb12 = nb12*bs*sizeof(half)/ts;
+        //nb13 = nb13*bs*sizeof(half)/ts;
     }
 
     if (need_f16_V && V->type != GGML_TYPE_F16) {
-        V_f16.alloc(ggml_nelements(V));
-        to_fp16_cuda_t to_fp16 = ggml_get_to_fp16_cuda(V->type);
-        to_fp16(V_data, V_f16.ptr, 1, ggml_nelements(V), main_stream);
-        V_data = (char *) V_f16.ptr;
-
-        const size_t bs = ggml_blck_size(V->type);
-        const size_t ts = ggml_type_size(V->type);
-
-        nb21 = nb21*bs*sizeof(half)/ts;
-        nb22 = nb22*bs*sizeof(half)/ts;
-        nb23 = nb23*bs*sizeof(half)/ts;
+        if constexpr (DV == 512) {
+            // DeepSeek. In this case the V cache is the same as the K cache, except that
+            //           it has 512 elements per row instead of 576.
+            nb21 = nb11;
+            nb22 = nb12;
+            nb23 = nb13;
+            V_data = K_data;
+        } else {
+            V_f16.alloc(ggml_nelements(V));
+            to_fp16_cuda_t to_fp16 = ggml_get_to_fp16_cuda(V->type);
+            to_fp16(V_data, V_f16.ptr, 1, ggml_nelements(V), main_stream);
+            V_data = (char *) V_f16.ptr;
+
+            nb21 = K->ne[0]*sizeof(half);
+            nb22 = nb21*V->ne[1];
+            nb23 = nb22*V->ne[2];
+
+            // Original PR in llama.cpp. Same comment as above for the K cache.
+            //const size_t bs = ggml_blck_size(V->type);
+            //const size_t ts = ggml_type_size(V->type);
+
+            //nb21 = nb21*bs*sizeof(half)/ts;
+            //nb22 = nb22*bs*sizeof(half)/ts;
+            //nb23 = nb23*bs*sizeof(half)/ts;
+        }
     }
 
     int parallel_blocks = 1;