From c61cc694137035c58c1d2439a4c44e0f924d41db Mon Sep 17 00:00:00 2001
From: Robert Morris <rtm@csail.mit.edu>
Date: Mon, 5 Oct 2020 06:59:33 -0400
Subject: [PATCH] eliminate virtio DMA into kernel stacks.

---
 kernel/defs.h        |  1 -
 kernel/virtio_disk.c | 20 +++++++++++---------
 kernel/vm.c          | 20 --------------------
 3 files changed, 11 insertions(+), 30 deletions(-)

diff --git a/kernel/defs.h b/kernel/defs.h
index 4b9bbc0..49aafdd 100644
--- a/kernel/defs.h
+++ b/kernel/defs.h
@@ -156,7 +156,6 @@ int             uartgetc(void);
 // vm.c
 void            kvminit(void);
 void            kvminithart(void);
-uint64          kvmpa(uint64);
 void            kvmmap(uint64, uint64, uint64, int);
 int             mappages(pagetable_t, uint64, uint64, uint64, int);
 pagetable_t     uvmcreate(void);
diff --git a/kernel/virtio_disk.c b/kernel/virtio_disk.c
index 070490a..cca44cb 100644
--- a/kernel/virtio_disk.c
+++ b/kernel/virtio_disk.c
@@ -64,6 +64,10 @@ static struct disk {
     struct buf *b;
     char status;
   } info[NUM];
+
+  // disk command headers.
+  // one-for-one with descriptors, for convenience.
+  struct virtio_blk_req ops[NUM];
   
   struct spinlock vdisk_lock;
   
@@ -219,19 +223,17 @@ virtio_disk_rw(struct buf *b, int write)
   // format the three descriptors.
   // qemu's virtio-blk.c reads them.
 
-  struct virtio_blk_req buf0;
+  struct virtio_blk_req *buf0 = &disk.ops[idx[0]];
 
   if(write)
-    buf0.type = VIRTIO_BLK_T_OUT; // write the disk
+    buf0->type = VIRTIO_BLK_T_OUT; // write the disk
   else
-    buf0.type = VIRTIO_BLK_T_IN; // read the disk
-  buf0.reserved = 0;
-  buf0.sector = sector;
+    buf0->type = VIRTIO_BLK_T_IN; // read the disk
+  buf0->reserved = 0;
+  buf0->sector = sector;
 
-  // buf0 is on a kernel stack, which is not direct mapped,
-  // thus the call to kvmpa().
-  disk.desc[idx[0]].addr = (uint64) kvmpa((uint64) &buf0);
-  disk.desc[idx[0]].len = sizeof(buf0);
+  disk.desc[idx[0]].addr = (uint64) buf0;
+  disk.desc[idx[0]].len = sizeof(struct virtio_blk_req);
   disk.desc[idx[0]].flags = VRING_DESC_F_NEXT;
   disk.desc[idx[0]].next = idx[1];
 
diff --git a/kernel/vm.c b/kernel/vm.c
index 0809aea..c99d2a5 100644
--- a/kernel/vm.c
+++ b/kernel/vm.c
@@ -121,26 +121,6 @@ kvmmap(uint64 va, uint64 pa, uint64 sz, int perm)
     panic("kvmmap");
 }
 
-// translate a kernel virtual address to
-// a physical address. only needed for
-// addresses on the stack.
-// assumes va is page aligned.
-uint64
-kvmpa(uint64 va)
-{
-  uint64 off = va % PGSIZE;
-  pte_t *pte;
-  uint64 pa;
-  
-  pte = walk(kernel_pagetable, va, 0);
-  if(pte == 0)
-    panic("kvmpa");
-  if((*pte & PTE_V) == 0)
-    panic("kvmpa");
-  pa = PTE2PA(*pte);
-  return pa+off;
-}
-
 // Create PTEs for virtual addresses starting at va that refer to
 // physical addresses starting at pa. va and size might not
 // be page-aligned. Returns 0 on success, -1 if walk() couldn't